There are many reasons why some men leave pregnant women. If the woman is his wife then it will probably be completely unexpected; after all he is committed to the marriage in the first place.
Other than the unexpectedness of a married man leaving his pregnant wife, it is usually pretty easy to spot the type of 'man' who would leave his pregnant girlfriend. 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 knowing him, you have to be aware that there are many potential dangers besides unexpected pregnancy. If you have been in the relationship for awhile, there have probably been many signs that this guy is a loser.
Women by nature are too easy 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 their boyfriends behavior. If you are this type of woman you should seek counseling so you can understand the consequences for this type of relationship.
The truth is that some men just do not like women and they keep on treating women badly. For some guys it can be a real ego boost to treat his girlfriend like a commodity. If you are in a relationship with this type of guy you can be sure he will leave you if you accidentally got pregnant.
Any man who is verbally, emotionally, and physically abusive will likely leave you whenever the relationship is not convenient for him. If your man shows any of these traits you should make sure that you do not accidentally get pregnant. Even if he does not leave you, he will become abusive and once you are with a child that abuse is almost guaranteed to increase.
So if you are with a man who does not treat you with love, affection, and respect, do not be surprised if you find yourself on your own if you become pregnant.