Determine the Gender of Your Baby in the Sims 3

It's all in the title- and generally simple!

Steps

Woo-Hoo (A Sim's favourite part of this process). To get your sim preggers, your relationship with the father must be at least a romantic interest. You must use romantic socials until it says, "Sim A think Sim B is extremely irresistible". Once you've gotten to this point, you're ready. Make sure there's a double bed somewhere in the lot they're on, and click "Try for Baby" with the father. After the Woo-Hoo is done, listen closely-- if you hear a little musical lullaby after the Woo-hoo, congratulations your sim's pregnant! If not, keep trying.
Increase the chances of a certain gender. Go to the grocery store at some point during the pregnancy. Buy three fruits, watermelons for girls, and apples for boys. Any more than three won't increase your chances any further.
Eat the three fruits. Any time before labor and after conception is okay. NOTE: The fruits will not always result in that gender of baby, but they will significantly increase your chances. Also, baking the fruits into food to eat doesn't increase or decrease your chances.

Find a sim that is higher then level 4 or 5 in the medical career. Either move them into the house or add them to the active family. Then choose to play as that sim.
In friendly interactions, choose Determine Gender of Baby. The sim will show a speech bubble and a picture of a baby and a female or male sign. Then in the top right corner, it will say whether your sim is having a girl or a boy.

You cannot determine the gender of your baby but instead you can try to get a baby boy or baby girl. By eating more apples, you more likely will have a baby boy. If you eat more melons, there are more chances of a baby girl. Eating more apples can also make chances of twins/triplets.