I don't get this...

what’s the deal with pregnancy? what is appealing about it to you people? i get some of it might be the whole “waifu” thing, but there’s still a fair amount of those pictures people enjoy of shipped characters, which i don’t get.

It’s a fetish. Don’t try to explain it, you will never truly understand it unless you have it.

Well like a lot of fetishes, it’s hard to explain/can’t be/something one just likes. However, being an actual function of humanity, there are reasons one would be turned on by seeing a pregnant lady:  
-Satisfaction. This is a reason my friend likes it, kinda difficult one to explain, but basically the same idea as the waifu thing.  
-It’s another curve to the body. A pregnant belly is not unlike the shape of the breast or buttocks. Men are hardwired to like the latter two things so, it’s an easy association. The human mind recognizes things by basic shapes.  
-Pregnant woman (usually second trimester) are more horny, or at least that’s how it’s usually seen.  
-breasts typically enlarge and lactate during pregnancy.
 
That’s the best I can explain it. I like pregnancy myself. Now, I fully get why it would be seen as weird, a baby in there and all that, but hey, baby is fine during intercourse in the womb, ain’t no dick or toy getting through that closed cervix.

If there’s a strange fetish out there that you’ve seen and you can’t even begin to try and understand it, then its in your best interest to leave it alone.

… Pregnancy? Of all the fetishes you could be confused about, pregnancy?

@Eggonaught  
…the one no one gets? the hardest to explain?
 
i could have picked vore. the “i want to get eaten” type of vore, because i don’t get that one, either.
 
i can see some. but this is the one that is the weirdest to me if the case is it’s not yours.

From a psychological point of view, it seems quite simple. Pregnancy implies fertility. Fertility is a desirable trait. Therefore it seems reasonable to assume that men find pregnancy attractive.
 
Whether that’s actually true or not, I leave up to the experts to answer. But it sounds nice.

Well what turns YOU on? Breasts? Butts? Vaginas?  
Try to analyse if your own relation to them is any more logical.
 
I agree with @Eggo that it’s an odd choice, given how much easier it is to tie this particular fetish to sexuality and biology.
 
There are all kinds of small attractions that draw one to a fetish, and people have all kinds of different reasons; you can’t measure them with the same yardstick. Someone might find a fetish empowering; another may like the innocence about it, or might experience it as a rebellios act; there may be all sorts of sensations that one is hooked on; and so on. What you think about sex is likely just the sum of your experiences and expectations too.
 
Eg. Jane the lesbian might feel it empowering and rebellious that she is doing it with a woman while bearing a child; Jack might feel a sense of accomplishment and ownership when he looks at his wife, and at the same time attracted to the body odours that come with pregancy.