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."
As this article on Balashon makes clear, mazal tov was originally used in the same way as we would say "good luck" in English - ie: an expression of goodwill for an event yet to occur. It was an expression, therefore, that was used when congratulating pregnant women, and not said after the child's birth.
In time, the phrase came to be associated with events that had already transpired, making its pronouncement inappropriate when the event had not yet occurred. In the words of David Curwin, whose article on Balashon I link to above:
And because it [the phrase, mazal tov] became associated with happy events like marriages and britot, it eventually transformed into a general phrase of congratulations. So you could say to someone mazal tov on something that had occurred entirely in the past. If someone won a contest – you could say to them “mazal tov”, even though there was no wish for the future. On the other hand, to say to someone who is about to take a difficult test or have a job interview “mazal tov” would be considered strange (even though you would say to them “good luck” in English.)...
After mazal tov made the transformation from wish to congratulations, a linguistic vacuum was created. That gap was eventually filled by the phrase “be’shaa tova” בשעה טובה, which really has the same meaning as mazal tov (literally "at a good hour"), and is used for something up and coming (like a pregnancy or engagement), but doesn't have the “evil eye” aspect of congratulating on something that’s already happened (which isn't an issue after a birth or marriage).
Curwin notes in that article that he has not found a pre-20th century source for the phrase besha'ah tovah being used in relation to an event that has not yet occurred. Saying it to a pregnant women would appear to be a very recent development in Jewish tradition.