When a person hears that someone is pregnant the traditional response is besha tova, instead of mazal tov. Why is that?
"Mazel Tov" is for something that's happened, like a wedding or a birth.
We recognize not to take healthy pregnancies for granted. To say "mazel tov" would imply "we're sure this will make it to birth", which sadly doesn't always happen. Thus, to show that it's in G-d's hands and not ours, we offer a prayer instead, "may the birth happen at a good time."