Make sure your sim has reached the point in their pregnancy where they have to change clothes, otherwise, most likely your efforts will fail.
Apples for boys, you only have to eat 3.
Watermelon for girls, eat 3.
For multiple babies (twins and triplets), activate the Fertility Treatment Lifetime Reward and, once again, wait until the Sim has changed clothes, then watch KidsZone Cartoon channel on your TV for a minimum of 6 hours.
As for having babies in same-gender relationships, you have to adopt. you can do this through any phone on your lot.