This achievement requires that you have a total of three babies. In order to have a baby, a couple must be married. After that, you need to purchase a crib and place it in their marital home. Click on the crib and opt to have a baby. It will take 24 hours for the baby to arrive via stork. You must have less Sims that your allotment, as the new baby will contribute to your population. Repeat two more times to unlock the achievement. Feel free to reuse the crib if you are cheap, even moving it to another home for a different couple. The same couple can have more than one baby. Also note that the babies do not need to be in baby form at the same time. As long as you created three total the achievement will unlock.