I assume that you never had kids of your own...
You have to have both sims in bed then press relax on both sims! Then the one that your not on press try for baby! Both sims have to be relaxing!then you will hear a weird sound and that means you will have your baby! But you have to wait a long time before she has a baby!!! You will notice that she is throwing up and her tummy is getting bigger than she will wear different clothes!
In other words, both must be intimate and cuddle to have intercourse to procreate.