While any two adults can WooHoo with each other, pregnancy can occur only when a male and female sim WooHoo. The start of a pregnancy is heralded by a characteristic melody. WooHooing is not guaranteed to produce a pregnancy, and the exact rate of success is currently unknown. Only female sims can get pregnant, although the male parent will share some of the child-related buffs.
Pregnancy lasts about two days. During this time the pregnant sim will have a gradually swelling belly, but the sim will walk as though heavily pregnant from the very first moments. A mother-to-be receives the "Pregnant" buff (+20 Focus) which ends with childbirth. Note that pregnant women are subject to some restrictions and problems:
Pregnant sims performing an activity receive the "Back Ache" debuff (-15 focus).
For the last few hours of pregnancy, the pregnant sim will automatically go home and wait there until their child is born.
Pregnant sims cannot drink from a beer/wine cask during the time they are pregnant.
A Sim cannot WooHoo while pregnant.
Gender can be influence with turnips and onions. Eating turnip soup during pregnancy means the sim is more likely to have a girl, whilst eating onion soup makes the sim more likely to have a boy. Keeping your sim happy (i.e: giving them food and sleep as soon as their hunger or tiredness bar goes down, and completing their assigned tasks for the day as well as spending lots of time with the baby once it is born) will make it more likely to have good traits, while doing the opposite will most likely give them bad traits