Setting: Secondary world technologically equivalent to the early 19th century.
Search terms: no new births, declining birth rate, sub-replacement fertility, historical pregnancy per capita, historical birth rate
My story takes place in a city of roughly 1.5 million people. A little over half of these people are natives of the country. The rest are immigrants ranging from first generation to people whose ancestors came over centuries ago. The newest ethnic group to arrive in the city - the Mirevans - is about 32,000 people strong. The Mirevans only started immigrating about 20 years ago as a result of a civil war in their homeland of Mireva.
What's a realistic number of pregnant women among these 32,000 people? How much chaos would it cause in the city if every pregnant Mirevan woman - regardless of what stage of pregnancy she was in - stopped being pregnant overnight as a result of weaponized magic? About a fifth of these women also die. Of those that live, maybe one in ten is rendered infertile.
It's discovered later on that the same thing has happened in Mireva, which has a population of about 2.5 million people. How badly would this disrupt their society? How much would the population of Mireva decrease over the next year or so given that there are no new births to replace the people who die each day? In addition to natural deaths, they also have to deal with an ongoing border war that kills off a few thousand conscripts a year, which would of course reduce the number of men available for marriage/procreation. The Mirevan faith doesn't allow polygamy or the marrying of people who aren't Mirevan so that would have to hurt population numbers as well.
Modern statistics help a little, but the better healthcare available today - at least in the developed world - also means longer life expectancies, less deaths in childbirth, and a lower infant mortality rate. Magic exists but there is no healing magic. Also the world is well into the age of quackery. Patent medicines - some of them lethal, many of them worthless - are sold on every corner and by pretty much every "reputable" alchemist/apothecary.
- Population decline