So this is the situation. I have 7 Sims in my household right now. One of my Sims is pregnant, so that would make 8. While she is pregnant I get the option to Move In her boyfriend. If I did that, then there would be 8 people in the household. What would happen to the baby? Would I be allowed to have it or would it result in a loss?

You wouldn't be able to move in the boyfriend unless you moved out someone at the same time. Sims doesn't have a mechanism for losing the baby - so the baby is considered sim 8 even though it isn't born. The boyfriend is actually sim 9.

Also, say you had 6 in the house and were trying for twins. Both parents have the fertility trait so twins are pretty much guaranteed. If you move someone in, they become sims 7 and you will only have 1 baby. When the game looks at number in the house, they only count a pregnancy as 1 sim to determine the house size.