If you leave a pregnant sim to play another household, the baby may be born before you return. If you leave a bassinet, you'll find the baby in the bassinet that you left. If your pregnant sim has twins, there will be another bassinet somewhere in the house. You'll be able to change the names that they were given when they become children -- just click on the name in CAS.
Your sims don't really exist while you're not playing them, unless you happen to see them in the park or on some other community lot you go to. So you don't need to worry about the baby being taken care of or whether the bassinet is blocking the refrigerator.
When you return to a household, you'll find all the sims from the household standing in front, probably needing to use the bathroom. I try to return in the middle of the night, so as not to make my sims late for work. However sims in the culinary, criminal, and entertainer positions do sometimes work then, so you need to be aware of that.