Female young adult and adult Sims in The Sims 3 can become pregnant following "Try For Baby" with a young adult or adult male Sim, or might even become pregnant after WooHoo.
Getting Pregnant
For a female Sim, becoming pregnant is fairly straightforward: choose the Try for Baby option with an adult male Sim. If the baby try was successful, a lullaby will play after the attempt. There is a chance that Try for Baby won't result in pregnancy, however.
There is also a small chance that just WooHoo will result in pregnancy, but this is less likely. However, young adults, and Sims who have had fertility treatment, have a greater chance of pregnancy from plain old WooHoo.
The number of babies (i.e. twins, triplets, etc.) is determined while the parents are trying for the baby, so getting fertility treatment while the mother is pregnant will not affect the number of babies from that pregnancy.
Elder female Sims cannot get pregnant, but elder male Sims can try for a baby with a young adult or adult female Sim.
Having a Bun in the Oven
When a female Sim is pregnant, she will give birth in three days. During her pregnancy, she automatically gets paid vacation from work.
In the first trimester (day one), the mother will suffer from nausea and will need to occasionally visit the toilet to relieve her morning sickness.
On day two, the mother will automatically change into maternity clothing as the baby bulge becomes visible. Other Sims can ask to feel the mother's tummy. The mother might get a craving to eat certain foods at this time.
On the third day, there are certain physical activities that the mother will not be able to do as her tummy gets too big for those things.
Giving Birth
When the mother goes into labor, the other Sims in the household will gather 'round and panic. The mother might decide autonomously to take a taxi to the hospital, or she might choose to stay home and have the baby there. The game will remind you to make sure that the mother is either in the hospital or at home. After a while, the mother will give birth, at which point you can name the baby and, if the mother was happy during the pregnancy, choose the baby's starting traits.
Getting Twins
Have the mother listen to the children's radio station on a stereo while simultaneously watching the kids' channel on TV. This will increase the mother's chance of having twins.
Choosing the Gender
If the mother eats three apples during pregnancy, she is very likely to have a boy. If she eats three watermelons, she is very likely to have a girl.