Answer Regarding Pregnant Women as Kvatterin

ANSWER:

When a woman is pregnant, the unborn child is also present, and counted. We see this with the מנהג by כפרות that a pregnant woman uses additional chickens for the unborn child.

Since the baby needs to be brought to the ברית by one man and one woman, a pregnant woman does not serve as קוואטערן, since that would mean that there is someone else escorting the child there as well.