for a long time, any mention of a lady being pregnant was really taboo.

pregnacy is proof you're having sex.

if you're unmarried, well, it could be a reflection on your character, (or a painful reminder of an assault)

if you're married, well, its a reflection of private communication between spouses. (and no one's business!)

things have changed--but to some degree, the less said the better. there are many things that can go wrong. mothers to be are anaxious, and self conscious. quiet congratulations are best, and then take the key from the M-T-B's reaction.

if she seem eager to talk, OK. but...

and to some degree, what ever you say, you say not just to the M-t-B--but to the couple (OK, showing a bias here.) since while the woman might be pregant, they are having a child.