Let's be honest; no one has the patience to wait for your sim to give birth. Thank god for cheats!

In the sims 2 You spawn the tombstone of L and D, click on it,more and then click "speed up my pregnancy"

The sims 3

I don't know about any right now.

The sims 4

Currently, the cheats don't work anymore. But if you're really dedicated you can download MC Command centre.