This is when a woman has sex with a man unprotected. The sperm from the man and the egg from the woman combine chromosomes to form a zygote, which becomes fetus (or foetus) to grow inside the woman's uterus, causing her uterus to expand, and in effect causing her stomach to grow. This causes swelling in other regions of the body, such as the breasts (which eventually begin to produce milk (lactation)), as well as symptoms such as increased hunger, mood swings, and pains in certain areas. Eventually, the woman goes into labor, in which the baby is squeezed from her vagina in a scene reminiscent of the chestburster scene from Alien. She then has to squeeze out the afterbirth, which is the liquids and placenta used for the nurture of the child.
This happens a lot of times annually, so soon enough, the world will be dommed, and most will die. In that scenario, feel free to eat your neighbor's horse.