There seems to be a custom of not saying Mazel Tov on hearing news or observing that someone is pregnant (there are even those that correct others that say Mazel Tov). Instead they say Beshaah Tovah, that the pregnancy should conclude at the right time.
Someone asked Why do we say besha tova instead of mazal tov? which addresses the reason behind this custom. I'm looking for a source, or the first time it appears in literature.