Instructions
Determine which gender you would like your Sim to have and then head to the grocery store. Purchase at least three watermelons for girls and at least three apples for boys.
Use the WooHoo feature and try for a baby. Your Sims must really like each other for this to be successful. Fertility treatments can increase the chance of having baby, along with having twins and triplets. Fertility treatments can be obtained by using 10,000 lifetime happiness points.
Confirm the Sim is pregnant before eating anything. There are two ways to determine if the Sim is pregnant. The first is children's music playing right after the Sims get done with WooHoo. If you did not hear the music, once your female Sim starts running to the bathroom with morning sickness, the pregnancy is confirmed.
Eat the three watermelons or apples, depending on which gender is desired, in succession.