Men often find pregnant women attractive because they are a walking billboard for fertility. Also, they have a healthy glow about them that is extremely appealing.
There may also something to this fetish that has to do with parental imprinting -- as lactating/pregnant fetishes are by far the most common in older siblings. This is probably do to the fact that we tend to find those like our parents attractive and if we remember our mom as pregnant or nursing...well, that gets imprinted too.
Guys can fetishize preggo bellies also because, well, she is obviously taken, pregnant and training to be a mother and all that. She is supposed to be good and chaste so fetishize that and you are being really 'wrong' and 'dirty' by making a mom-to-be into a sex object. That combines with the fact that she is already taken and bam, there-you-go!
Pregnancy can also be strongly associated with a time where the woman is unspeakably horny (this is a biological attribute that keeps the father around and interested)... but a sex-crazed-horny woman is always of interest to men. Additionally, I have also heard that pregnant women have their tits get bigger...something I have not yet been able to definitively verify in person as of yet and more evidence is required.
And speaking of being of interest to men. a
pregnant woman has a body that is constantly changing. Her body is constantly doing new things and altering its form...making her a source of constant fascination and wonder.
I know that women often feel like they are a bloated hippopotamus and as less attractive because they have just gained 40 pounds in a society fixated on being thin...but pregnancy is not being fat: it is being fertile and performing a function that is integral to the continuation of the species. It is a god-like power to bring new life forward. And being powerful is always sexy.