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Male (human) pregnancy - why it wouldn't work
Del Rion illusion
del_rion wrote in little_details
I'll try my luck in this community because web searches have failed me and this is an interesting topic in a sense...

(The info is going towards an Alpha/Beta/Omega verse with a lot of twisting and world building...)

The story takes place in a modern setting. Prior to the current events a mysterious "illness" passed most of the human population and there have been changes. (The changes are still going on, slowly, so what happens at this point is physiologically a work-in-progress, not a finished product - whatever the finished product may be.)

To the actual question:

The basic idea is that even in the verse, men should not be getting pregnant - it's a mistake, and thusly their bodies are only half-way prepared for it. But, if and when they do get pregnant, what kind of problems might there be?

I've been trying to find facts on why men would not be able to carry out a pregnancy (skipping the obvious reason of missing equipment - that is being bypassed in the story). I'm looking for less obvious things, like the strength of a male heart versus that of a woman (a friend of mine thought she saw it somewhere that a man's heart could not pump enough blood for both him and the baby, plus my MC already has heart issues so that will be a problem). What about other organs? How good are they at re-arranging themselves to make room for a growing baby? (No actual births are taking place at this point so the beginning months/first half of a pregnancy is targeted by this question...)

Used search words have ranged from "male physiology", "male pregnancy" to "why men can't get pregnant" and just about every variation I've managed to think of  to bring up a response from Google.

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Insufficient progesterone and oestrogen to get or stay pregnant? Some women have to take progesterone supplements in pregnancy to avoid miscarriage and, IIRC, they are mostly produced by the ovaries, which men don't have.

Hormones cocktails which transwomen take... in more concentrated, carefully monitored doses with a doctor on the stand-by in case of complications...?

I'm not a medical expert in any shape or form, but I think I heard somewhere that a woman's skin allows for stretching out a lot over a relatively short time, and men's doesn't. Not sure if it's really true, but it might be an avenue to explore? Maybe someone will know more about this,

men have beer guts... some as large as full-term pregnancies. so stretching of the skin is not a problem...

So, check out the picture on the right hand side of the Wikipedia Article on the human pelvis. They're incredibly different, and the space between the bones is much larger (duh) for women than for men. Also the connective tissue is different and the way the bones connect at the pelvic bone is very different, in order to allow for the softening and stretching of ligaments during birth. A male pelvis would have a hell of a time allowing a baby through it, even given a male uterus in an A/B/O world. I'd suppose that without a whole lot of evolution to change the omega pelvic structure they'd probably need a C-Section to birth a baby -- you could make that as risky as you needed to, depending on how you designed the Omega anatomy.

Men have bigger lung capacity and higher hemoglobin, so as far as O2 transfer to the fetus, they'd probably do better than women. (Lucky sods, I hated being breathless for the last 6 months of pregnancy...) But they do have lower body-fat percentages, which would make it harder to sustain a pregnancy, because there are less reserves to draw on.

Re: the pelvis issue, you could just go whole hog and have humans develop a sort of marsupial arrangement -- the baby is born tiny, blind, and only weighing a few ounces, then carefully deposited into a pouch where it spends the next few months suckling and growing.

Not gonna lie, I'm kinda pissed our species didn't do that anyway. You know how many women would've been spared deaths in childbirth if we didn't have to push a gigantic head through an itty-bitty opening? Also, kangaroos can put a fetus "on hold" for however many months if conditions aren't right, and then give birth when they're ready. WTF nature, just give us a glimpse of what we could have and then laugh in our faces, why don't cha.

There are several things I can think off. (Those are all on average, though, variation between individuals also matters a lot.)

One is chromosomal sex (I assume your men to be both XY and impregnated by other XY men). With two XY parents, the chromosomal sex distribution in offspring is: XX (25%), XY (50%) and YY (25%). YY fetuses are not viable as the X chromosome carries a variety of necessary alleles and the Y chromosome is virtually useless, being much smaller (iirc, there's only one gene it that does anything, the SRY gene).

The other is hip shape and size. Women's pelvises are shaped differently than men's, to make it easier to bear children: they're wider, to give the fetus more space, with a wider pelvic opening and inclined at a different angle to facilitate childbirth.

Women's spines are also shaped differently: the third lumbar vertebrae is wedged to help spread out the load of the fetus.

Women's hearts beat faster (iirc, about 15 beats more/minute) and pump up to 50% more blood volume during pregnacy, which can cause tremendous strain to the heart, as well as preeclampsia*.

In some cases, the fetus is viewed as an invasive body (especially in ectopic pregnacies**) due to the genetic material being different to the mother's and are attacked by her antibodies. I expect this to be more likely in male pregnancies.

*pregnancy-related hypertension
**Pregnancies occuring outside of the uterus

In general, I'd suggest looking at compliactions of pregnacies and extrapolating from there.

Sorry for all the edits, I can't seem to brain right now.

Edited at 2014-02-17 10:53 am (UTC)

How exactly does a man, who is biologically male (that is to say, the carrier of XY genes that have expressed in the normal ways throughout his development---basically, a man who is not a transman, and thus does not possess an intact uterus) get pregnant in the first place?

I'd venture that, for some reason, your illness would involve some kind of virus/bacteria that a) stimulates the growth of at least ovaries, of not a uterus and fallopian tubes, in a human male, and b) introduces some way for the male body to start manufacturing the hormone cocktail needed for ovulation (FSH, estrogen, progesterone). There would have to be a way for sperm to come in contact with the ovum. He would probably develop breast tissue as well, and perhaps be able to lactate.

If your illness does not cause a man to grow a uterus, the pregnancy would then have to be ectopic, where the fertilized egg implants outside the uterus. Ectopics usually happen in the fallopian tubes, but have occurred on the outside of the reproductive organs, the bladder, the bowels, and elsewhere in the abdomen. Because these other sites aren't equipped with a nice cushy blood-filled endometrium, the pregnancy cannot progress normally. They will make enough hCG to trigger a positive pregnancy test, though.

In addition, pregnancy puts strain on the liver, kidneys, lungs, and heart. Blood volume increases 30-50%. I have no idea if male organs are unable to adapt to the extra load the way women's are. Also, male pelvises are not made for birthing, so if your men do grow uteri, you're looking at a caesarean birth.

If your character has preexisting heart issues, that would definitely be a factor. Women with certain heart conditions are discouraged from getting pregnant. Ditto if he has any kidney issues.

Or you could completely suspend the belief in reality and watch Arnold's m-preg movie starring Emma Thompson, "Junior".

I doubt the heart thing is true.

The biggest problem is that a womb is pretty good at keeping the mother from bleeding to death during pregnancy and delivery, and even so, tons of women die during childbirth without proper medical care, and have since, well, since our species evolved. A dude (excepting transdudes who haven't undergone hysterectomies, etc.) doesn't have a womb, isn't going to produce the proper hormones to keep his immune system from attacking the fetus, and the embryo is going to have to attach itself via placenta to some organ or another in his body -- basically like an ectopic pregnancy. Intra-abdominal pregnancies do happen IRL, where the fetus attachs itself to, say, the liver or somesuch. Occasionally, with good medical care, all is well. Usually the fetus doesn't make it. Sometimes the mom doesn't make it, either. I'd say any kind of practical male pregnancy (with medical assistance, natch) is still decades away.

Assuming some sort of Alien Space Virus that is mutating humans, I suppose you could handwave it by saying that the virus is causing human reproductive organs to transform into some intermediate form -- like how testes are the counterpart to the ovaries. The guys still have to develop some kind of passage that will allow sperm to reach eggs and fertilize them.

In addition to the pelvis not being wide enough, it's also not strong enough to support the weight of a growing fetus; at some point there may be a risk of bones breaking.
(Unfortunately, I don't remember where I read this, so I can't guarantee it's correct.)

I don't get how a man's pelvis couldn't support a few extra pounds of weight. I mean, plenty of people carry dozens or even hundreds of extra pounds on their frames and their pelvises don't snap. My boyfriend can easily pick me up and carry me about over one shoulder, I can't imagine 7-8 additional pounds in his pelvic area could break any bones. I mean, yeah, the spine wouldn't be happy about it but human spines got a fucking raw deal anyway.

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Hormones. It' not just a matter of a sperm hitting an egg, the pregnancy has to be sustained for nine months of intense work, and male bodies don't have the programming to guide it. Progesterone and oestrogen are the best known, but there are more such as relaxin - produced in both males and females, but in males it's produced steadily along with sperm while in females it varies all over the menstrual cycle and during pregnancy.

Also, testosterone is a teratogen. Teratogens during the first trimester generally cause spontaneous abortion (miscarriage).

I'm sure I read something years ago that said it would be theoretically feasable, but I have no idea where I read it. Essentially you had to implant the foetus at the same stage as in IVF onto an organ with a good blood supply, eg the liver, and do a caesarean as soon as the baby is viable.

I'm still struggling to come up with a sufficiently helpful response to this, since the assumptions being made about biology - even in an SF context - here are really problematic.

But meanwhile, here's a relevant NSFW comic for your entertainment.

Oh, and this little class-A memetic hazard, covering both how this has been done before and why it's potentially a bad idea.

As to my problems with this enquiry:

1) It isn't clear to me as a lay reader what the constraints of the 'verse you're talking about are.

2) The idea - implicit in the question and explicit in at least one answer - that there is such a uniquely-defined thing as a 'biological man' is unscientific, trans-erasing and unhelpful.

3) You have assumed your conclusion in your header; having asked it as a closed question, you are getting well-worn urban myths instead of actual data.

Women's center of balance is in the uterus so that the baby doesn't throw them off balance. A man's is much higher.

Won't make it impossible; will make it very difficult to manage.

If you haven't looked up Thomas Beatie already, you might be interested. He is a transgender man who was born female and had a double mastectomy, but because his wife was unable to bear children, he impregnated himself through artificial insemination and bore three. I don't know if the hormones he took for his transition would have affected the babies, but you may be able to find some more relevant information on it.

Mr Beatie stopped taking his male hormones in order to become pregnant and sustain the pregnancies. Once he was done, he returned to taking them. There was no apparent effect on the fetus.

OP said: "...why men would not be able to carry out a pregnancy (skipping the obvious reason of missing equipment - that is being bypassed in the story)..."
Please bear with me as I back up on this statement for a minute. I think this is skipping the most interesting part of the whole universe.
A lot of Alpha/omega stories magically handwave as if they assume the necessary parts are in fact there and functional. Such a viral change has already happened in those stories. You already have weird biology, you could come up with some really cool biology which you as the writer could take some fun advantage of, plotwise. Unexpected medical twists, for instance.
I suspect the impact of a successful viral attack is far more likely to be fuller development of existing female organs to make an omega reveal they are in fact hermaphroditic,or hormonal realignment of a genetic female who was masculinized by prenatal hormones into appearing male when they aren't. In that case the organs are female and may be deformed or poorly functional. Rejecting the fetus in a miscarriage is likely.
That brings us to the marsupial suggestion. You haven't specified much about the level of tech available to the general population. Current medicine hasn't been able to support a human infant in such a premature stage. Their lungs don't work well enough to support the developing brain when they're born too prematurely, their immune system is too easily overwhelmed, etc. Obviously a kangaroo joey has a fairly demanding brain development too, so it's not entirely impossible that an omega could revert much, much farther back to some ancient genetics inherited from a marsupial mouse or something of that sort. The likelihood would be much higher that the viral attack giving such extreme genetic derangement would result in humans suffering high death rates from fast-moving horrible cancers and that ordinary births would be higher risk, lots of really bad birth defects.
If you decide on internal fertilization with a full pregnancy as a result of the viral attack, you're saying in your story the viral assault has just started picking out survivors. These would be parents who do manage to generate necessary blood supply, they manage to support the fetus safely in amniotic fluid, to produce the necessary pulses of hormones, to suppress immune system recognition of the foreign genetic body (which some women don't entirely successfully manage to do either), and so on. It's an important question whether survival depends on surgical intervention, or whether some kind of aperture for a birth canal is developed.
I have read intriguing suggestions that eventually medical intervention could implant fetuses in men inside the mesentery, attached to the large intestine, which has a decent blood supply and would allow the pelvis to support the infant in a way it would not be supported from the liver, which is higher up. This does not lead to "ass babies," the baby is embedded in the abdomen "outside" the tube of the colon. Obviously repeated surgical intervention and suppression of immune system attacks would be necessary, which creates questions whether those drugs would also suppress the infant's developing immune system. In addition, there is no incentive for the placental blood vessels to separate from the intestinal blood supply in the same way as the uterus's hormonal methods do, so either that has to be applied artificially and surgery is done to clean up the leftover mess, or it's entirely a surgical intervention with a great risk of hemorrhage.


OK, I'm going to ask you, since the OP hasn't explained this - what are 'alpha/omega stories'? What's the setting/fandom? TV Tropes has (strangely) been no help here. I still feel like important information is missing about the setting's existing assumptions about how male childbearing might work.

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Hello, MD here. tl;dr: Honestly, there's no reason, given the assumptions of some sort of an A/B/O AU. Use a regular dangerous pregnancy reason.

Pelvis shape is not relevant. True, the shapes of pelvises are called "android" and "gynecoid" and one is supposed to be better for getting a baby through the outlet than the other, but basically as many women have the android shape as gynecoid, but sexist anthropology has erased this. Take it from my good friend the forensic anthropologist: sex cannot be definitively proven based on pelvic shape. Strictly speaking, the shape of the pelvis, or rather more importantly the diameter of the outlet (where the head must pass) is only a problem for getting the baby out through the pelvis floor, that is, through a vagina, or in this case maybe anus. Some women with narrow outlets have trouble giving vaginal birth, and these women died in the past. Cesarean makes this problem moot for women with access to medical care, as it would for our hypothetical man.

As to weight bearing, there's no problem with shapes of pelvises and weight bearing (as our wide variety of shapes and sizes shows) except maybe for degeneration of knee cartilage in the overweight, certainly not for bearing a child. Haven't you seen a many with a beer belly that could well be a pregnancy? I have! And the incredibly rapid onset of ascites in a person with liver failure of either sex gives a pregnant weight distribution and appearance much faster than an actual pregnancy.

Hormonal milieu is key for maintaining a pregnancy, true. But within an AU where a man ovulates and gets pregnant, surely this is handwaved with special organs or neurochemistry or hormones or whatever? Men and women share all the same "programming" and "genetic code" in all their tissues that people have been referring to, but the difference is the current milieu and also the triggers for action. I have tons of estrogen and progesterone in my blood that vary to a specific pattern, produced by my ovaries in response to hormones from the pituitary which themselves are directed by the hypothalamus. But in fact the higher-level hormones are the same in men and women (GNRH, LH and FSH) but lead to different results because the ovary is stimulated by them to produce different effects than the testes are. Once the sex hormones are produced though, their effects are standard. Excess estrogen in men with cirrhosis of the liver leads to breast development, among other effects, and if I took testosterone, the muscles in my shoulder girdle, carrying XX though they do, would respond to the excess testosterone and bulk up, and I would grow more hair, and more acne, and my body fat distribution might change some. All fat tissues and hair tissues and muscle tissues, etc. etc. respond the same to the same hormones, it's just a matter of what hormones are about. Which leads me to the hormones required for pregnancy:

As a woman, if I were to get pregnant, a normal corpus luteum left after an ovulation would produce progesterone enough to maintain the pregnancy (that is, keep the wall of the uterus receptive and helpful in keeping the embryo going) until the placenta was developed enough to take over through the related hormone, hCG. From then on the placenta produces all the hormones that make the changes in a woman's body described by other commenters: skin stretching, the loosening of the pelvic symphysis, the huge increase in blood volume and slightly smaller increase in hemoglobin that taxes a woman's heart. Given some place to implant an embryo and some organ to produce the right hormones (ovary, or maybe a magical multipurpose testicle) all the same changes would happen to the heart and cartilage and hematopoietic system of a man.

(continued)


For the heart question in particular: provided the person is healthy cardiovascularly, either a man or a woman would adapt as easily to the increase in blood volume. Those with heart failure or congenital or acquired structural defects do not adapt well, and that is one of the ways pregnancy can kill a woman (or hypothetically a man). But given the same stimuli a man's or woman's heart would react the same.

The commenter who mentioned lung capacity and hemoglobin is right, but specifically this is due to greater average body size for lungs and the effects of testosterone on hemoglobin production. So a small man has less lung capacity than a large woman, but most men are larger than most women. And women actually increase their total body hemoglobin by about half in pregnancy anyway, though the increase in total volume (water) is greater so their average concentration drops (normal is around 10 in a pregnant woman.)

For body fat, that's easily changeable by eating a lot. hCG induces a special kind of pregnancy-diabetes (sometimes enough of it to be clinically a problem, thus gestational diabetes) in all women that raises their blood glucose a bit to provide for baby. There's a lot of work being done on these hormones do to with weight gain and loss, but suffice to say the hCG diet is worthless.

The chromosome problem (man with XY plus another man with XY yields 25% YY nonviable fetuses) to me just says that you'd have a lot more early miscarriages due to chromosomal abnormality, but it's thought that up to 25% of real-world pregnancies end in early miscarriage, many of them for this and other reasons (XO, XXX, XXY, XYY, as well as autosomal abnormalities). We are very messy creatures, what can I say.

As for testosterone being a teratogen, it is... for female fetuses, and it acts to masculinize them. But it's not a teratogen in the lay sense that it would kill or hurt the fetus, just alter its development, and even then only in the genitalia and in the wibbly-wobbly unknown way of neuroscience. But given that the assumptions of the question predicate an ovulation and maintenance of pregnancy, which requires literally grams of estrogen produced daily by the placenta in the later trimesters, I think the man's testosterone production could very easily be handwaved, or just drowned in the sea of estrogen.

Our immune systems are not any different, men or women. The immunologic tolerance needed for pregnancy is created by the placenta's barrier between the maternal and fetal blood supply, which prevents the baby antigens from being exposed to mother's immune system. Abruption, miscarriage, fetal bleeding at delivery, and fetal bleeding at ruptured ectopic etc are ways mother is exposed and require Rhogam to protect subsequent fetuses from hemolytic anemia. Given a placenta, a male pregnancy would be just as immunologically protected as a female one. There are also immunosuppressive effects of the hormone load from the placenta, which is why mothers with MS often have remission of symptoms and one of the several reasons flu is so dangerous for pregnant women. Again, it all comes back to the placenta!

(continued yet more)

Finally, with regard to where the embryo implants: there have been ectopic pregnancies carried near enough to term for the baby to survive, though obviously they must be born by C-section. Not common, and the most common place for an ectopic is in the tube, which will rupture when the pregnancy gets to a certain size, leading to bleeding etc. But ectopics on the surface of the gut or mesentery or the serosal surface of the uterus or wherever are feasible. The things that the embryo has at its disposal to invade tissue and recruit blood vessels and rebuild a person's body to support itself are actually pretty terrifying, and they work pretty well. Again, requires a C-section, and I would guess there would be more danger of placental abruption since there isn't a nice warm cave of muscle to surround it.

I'm sorry this got so long. I think I was riled up by the idea that men's hearts and bones and skin and whatever are innately different than women's--not true. Basically all sex differences are either caused by exposure to hormones in the womb, leading to taking the mullerian path or the wolffian for the internal/external genitalia, or by our own hormones in puberty. Which is why giving trans youth puberty suppressants works so well, because if given the "right' set of hormones during puberty you can basically build the "right" secondary sex characteristics, muscles, skin, bone, voice, hair, breasts or lack thereof and all. All of the preceding was mostly rebuttal, sorry.

To the OP: If you are designing a universe where men suddenly can ovulate, complete with progesterone production from a corpus luteum, and can expose the ovulated egg to sperm, then all of this is possible. Once you have an implanted embryo that can recruit blood vessels to support itself and build a placenta, all of the normal pregnancy changes will happen, including breast tissue hyperplasia.

I think your best bet would be to have the lack of a uterus be the trouble and the pregnancy implant in the peritoneum somewhere, like the mesentery or the gut wall. How to get the sperm there is up to you*: special new organ growing off the rectum, perhaps? Gastroenteritis that makes the colon wall leaky? (But who would want to do anal at that time?) The trouble with those for me as a reader is that they'd immediately get infected like a diverticula might, but I'm a specialist. So handwave it. A retrovirus might make a testicle go rogue and express different genes and suddenly behave like an ovary, but inducing a uterus to grow would take a mad scientist designed army of viruses. Unless that's what you've got?

Then the danger to your character is twofold: 1. Tenuously connected/connected to insufficiently vascular bed placenta makes abruption and miscarriage more likely. Bedrest? Is he going to get punched in the belly a lot? This is a danger to the baby; is this a wanted baby? 2. If you abrupt a placenta, the host (man or woman) can also bleed to death, particularly if the cavity he is bleeding into is the peritoneum which can take liters before it is full, whereas a uterus contains and can tamponade bleeding before she bleeds to death. For reference, in normal pregnancy, abruption is host bleeding that intervenes between the placenta and where it has implanted, which decreases supply to the placenta. The cause is ripping of the vasculature between the two.

Or you could have the pregnant person have one of the myriad conditions that make pregnancy dangerous: lupus, Marfan's, congestive heart failure, influenza in the 2nd or 3rd trimester, etc; or have him get pre-eclampsia, which is caused in a roundabout way by all the hormones that the placenta makes, which is why delivery is the only cure (remove placenta, remove hormones).

Whew. Sorry. Or thanks? Not sure which one. Hope that helps.

*One of my favorite papers getting around this problem in a woman with no vagina.

This was very interesting, thanks!

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* http://www.ncbi.nlm.nih.gov/pubmed/3191066

I don't think it would be impossible at all, as long as you have *something* to keep the hormone levels correct throughout the pregnancy. People aren't all that different. The pelvis *tends* to be slightly wider and tilted in women but there's huge overlap - some men have a more "womanly" pelvis than some women, and that's the same with most physical issues - e.g. men tend to be slighter taller and more muscular than women on average but that doesn't tell you much about any individual man or woman. Some occasional men can even breastfeed.

It is a lot easier to grow a baby in a uterus, but women do have (and these days sometimes survive) extra-uterine pregnancies. The only other real complication I can think of is that two men would have a 50% chance of a male infant, 25% chance of a female infant and 25% chance of a most-likely non-viable infant (with some variation for alternative chromosome arrangements).

A YY embryo is non-viable. IIRC the embryo will only divide a few times and then die. It will never make it to be a fetus. There's not enough genetic material on the Y chromosome (sort of like trying to build a house when the only blueprints you've got are for a front door and some cabinets).

It's a complete suspension of belief, but you might want to refer to that movie with pregnant!Arnold Schwannegaar (dunno how you spell it) and Emma Thomson, "Junior". They pull facts outta their asses, but its convincing in their scientific mumbo-jumbo like Arnold taking female hormones throughout pregnancy - the same kind recommended for a pre-op transwoman, but in more concentrated doses at frequently moderated intervals. The fertilized egg is medically implanted in the hollow pelvic cavity beside the stomach... which would give it an appearence of a very very prominent beer-gut or look oddly grotesque in case of a smaller man. The question about pelvic-adjustment could be explained away prolly because Arnold is a huge guy, but in one of the m-preg fics I've read, the pelvic bone cracks during the third trimester to adjust to the growing foetus.

about the heart - well, woman's hearts are a bit stronger - but there's always dieting and exercises to regularize that, and complete bedrest in case all else.

Unless suppressed by medication or the "disease" process, a man's sex hormone levels will be very problematic. Even if he has enough estrogen and progesterone to support the pregnancy, his testosterone will be high enough to affect fetal development. It's more obvious if the child is a girl; if she's exposed to too much, it can seriously affect the development of her own sex organs and she may even appear male at birth if it's extreme enough. Baby boys already look like baby boys if nothing unusual happens, so I'm not sure what the effect on them would be, but it's probably not good.

To be honest, if you manage to create an environment for the baby to grow in, get an egg to create a baby, and manage to supply appropriate hormones to maintain the placenta and pregnancy, you've pretty much solved the problems in order to make the pregnancy happen at all. One option would be insufficient hormone levels to maintain the placenta and thus the pregnancy fails at some point that you decide.
Other than that, look up pregnancy complications and pick some - possibly the cocktail of necessary hormones had side effects like really bad vomiting or faintness so the pregnant man has to be on bed rest or a drip. And relaxin is a pregnancy hormone which enables the female pelvis to spread enough for a baby to come out. If a male pelvis were going to adjust that way, the pelvis would get so weak in pregnancy the man would be unable to walk and need a wheelchair for months - look up SPD or pelvic girdle pain.

Well, they could just end up with literal ass-babies, and then have...idk, ruptured intestines, or blockages from having many pounds of baby stuffed in their ass. Or just dead baby, because I can't think of a shittier environment for gestation (pun totally intended).

Unfortunately gross, but definitely a hindrance.

Or really, why make it the Omega's body changing when you can have the Alpha's knot actually be a brand new egg-depositing organ? Massive loads of cum, or billions of tiny fertilized eggs that will eventually overtake their host? Both are sort of non-scientifically valid!

Might be too dark. Depends on what you're writing.

Human beings can be born with all sorts of intersex conditions, but one thing you don’t see is someone with fully functional male and female reproductive systems in the same body. The embryonic gonads can grow into testes or ovaries, or not grow at all, but they can’t become both at once.

If your alt-biology allows for people who can both generate sperm and become pregnant, then you’ve already pretty much thrown out the rulebook for human biology and you can replace it with whatever you want. You can read about the mating habits of gastropods and earthworms for inspiration.

The Wiki article on male pregnancy is pretty comprehensive, including on those men who have 'carried' an ectopic twin to (caesarian) 'birth'.

I suppose the soft fantasy option of shifting into a female or hermaphrodite body, or at least changing a few organs, doesn't fit this 'verse?

I'm aiming for something gritty and "realistic" with this one, so the changes are... well, not at all natural, of course, and most likely not possible in real life, either, but I'm trying to tether as many things as I can into real things, so, shifting and an uterus magically appearing are out of the question ^^

(that would be convenient, though... and less painful, for sure.)

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