If you have wondered, hopefully not from personal experience, why men leave pregnant women there are many reasons. If the woman is his wife than it will probably be completely unexpected; after all he committed to the marriage why bail now?
Other than the unexpectedness of a married man leaving his pregnant wife, it's usually pretty easy to spot the type of 'man' who would leave his pregnant girlfriend. I think all women need to use common sense before they get intimately involved with any man.
First of all if you are entering into an intimate relationship with a man before you have really gotten to know him you have to be aware that there are many potential dangers besides unexpected pregnancy. If you've been in the relationship for at least a little time there have probably been many signs that this guy is a loser.
Women are too quick to 'forgive' bad behavior in the men they date. They will say things like "he's got a good heart', "that's just the way he was raised", "it's a guy thing", etc. to justify the selfish and inconsiderate things their boyfriends do. If you're this type of woman you really should get counseling so you can understand why you would settle for this type of relationship.
The reality is that some men just don't like women and they get off on treating them badly. For some guys it can be a real ego boost to treat his girlfriend like trash and have her keep coming back for more. If you're in a relationship with that type of guy it's not too much of a stretch to think he might leave you if you got pregnant.
Any man who is verbally, emotionally, and especially physically abusive will more than likely leave you whenever the relationship isn't convenient for him. If your man exhibits any of these traits you should make darn sure that you don't accidentally get pregnant. Even if he doesn't leave, he is already abusive and once you are 'tied down' with a child that abuse is almost guaranteed to increase.
So if you are with a man who doesn't treat you with love, affection, and respect, you shouldn't be surprised to find yourself all on your own if you become pregnant. I'm not sure why men leave pregnant women expect that the term 'man' is probably not a very accurate description of these types of guys.