I'm going to say third trimester for the reason that when most baby's are born their heads come out first in most cases. For the most part, they travel down the birth canal head first. If that doesn't happy the doctor tries to position the baby so that it won't be a breached birth meaning the legs come out first. By the head coming out first the baby is able to have an air supply where as with a breached birth that doesn't happen.
Hopefully this helped and good luck.
