Supposedly it's due to primative instincts.

A women being pregnant shows she is currently fertile, thus increasing the chance that she will be again when you, well, have your turn. Thus, allowing you to procreate.

What comes to evolution hypothesis: In other words, we (maiesiophiles) may be more or less chosen by these genes to be more primal, something blast from the past, something ordinary/original than most of modern people (men)... which is fascinating  in some way. Cursed by ancient genes (which cannot be suppressed), and at the same time - well, l think l made my point.

Remember Venus figurines?

I may have a possible psychological cause as per why us preggophiles, or rather, most folks who have a paraphilia in something manifests itself in a good, pleasurable way, and that it's due to a known phenomenon in psychology called conditioning, which is essentially about doing or exposing yourself to a stimulus with an associating reward enough times and even after the reward was long since removed, the reaction to the initial stimulus will still be same, or should be almost the same. Having said that, if a child was to be exposed to pregnant women while being in the formative stage of life where they learn how things work and such, like the joy of youth, if enough exposure was given, the child learns to associate pregnancy with happy times or when they're discovering themselves and the world around them, which is often one of the prime catalysts as to why some people would eventually develop a curiosity or in this case, intimate fascination with pregnant women.

Well I know for myself it probably has to be when I was playing house or seeing my aunt pregnant in the early 2000s. From there it just evolved into trying to understand it and what to expect. While I've been at work for the past 3 years at my local Target as a cashier I had to suppress it to be more professional and treat them like any other guest. It's more about what happens to you personally than professionally. I've had a 9th Grade Teacher that was pregnant and she made my heart race and I wanted to be in detention with her to just rub her belly. One co-worker where you want to do that break room sex thing. But looking back on it it's suppose to be how do rip apart fantasy from reality when you are with one. But the others here have better advice and explanations on it than I do.

Hope this helps

ln addition to previous replies about hypothesis why fetishes begin, there is also an aspect which doesn't connect pregnant fetishism to being horny or reproductive, but feeling save etc., like Natallie-twinner got in messages:

hyyps://www.youtube.com/watch?v=6A0ABvgqs3Y

For example my fetish is usually somewhat random (normally l don't remember about it, unless l randomly remember that l have it by accident or when seeing a public preggo. Then check videos and/or pics, do some artwork etc. l don't know why l collect videos and pictures, my mind just tells me to do that, even thought they sit in the folders like 99,9999% of time), but when l get strong personal crisis/loss (for example like losing mother in early age), l cannot stop thinking about them for a day for some duration of time (l don't remember last time, but now it has been several weeks). My body may say that you get somewhat horny about them, but my mind doesn't do that, and l don't have need to "satisfy" it by myself, or anyone in that way. But anyway, l've had this since l was a teenage. l feel lonely, maybe this fetish is a part of getting some secure feeling (but not necessary limited to), which l cannot get easily in my part of world.

An interesting topic and one where I can only echo the comments already made by others.

I think Jacen101 more or less said what I wanted to say perfectly. In short, I'd put it down to the extenuation of attractive curves - with of course, the belly becoming one enormous curve. It's a natural instinct for men. A primal instinct.

I can only imagine our personal senses, having the pregnancy fetish, makes that instinct stronger than in others.

In my case, it's the idea of fertility and life-fullness a pregnant woman conveys, but also the male part in the process. I'm bisexual and have always had a huge turn-on for both pregnant women and fathers-to-be. The sexier and bigger a pregnant woman is, the more curious I get to see who made that masterpiece - I'm perfectly aware that conception involves both male and female fertility, and that a bump's size has little to do with male potency, but I just can't help to consider a huge bump as a signal of sperm power.

As someone else who is bi, I completely understand this.

Because pregnancy suits almost every woman. I don't believe anyone who says they don't like pregnant porn. For example, I don't like skinny women at all. I've never watched a Kate Rich video in my life. But pregnancy suited this woman well and she made amazing videos. I loved the fast Deepthroat and Gangbang scenes. Another woman, Kyra Sex, was amazing too.

I have thought long and hard about this over the years. At first I did not think it made much sense, and was difficult to explain, but now I think it makes perfect sense and is very easy to explain.

On a Personal Level:
I think this fetish began to develop for me a few years before puberty, I had a couple of very vivid dreams about people or myself becoming very fat and immobile, but I did not really have any sexual link to that, being that I was too young, so these dreams were very confusing.
Then at school I noticed a couple of teachers become pregnant, and became very interested in how big their belly's grew, and how people treated them differently. There was also some literature like 'Goodnight Mr Tom' which went into detail about the main character becoming intrigued by his pregnant teacher which really struck a cord with me. After that, and going through puberty, with the rise of the internet at home and late night TV, I sought out ever bit of pregnant erotica I could find. I kept this all hidden for a very long time, and was very embarrassed and ashamed of this kink, mostly as I think some people get a bit confused as to why people find pregnant women attractive, and seem to automatically flip to the worst possible reason (which also does not make any sense). Not that I am really that embarrassed or ashamed of this kink anymore, but these feelings of embarrassment and being ashamed have now kind of worked their way into the kink regardless.

Generally: In order of general relevance.

1) It's nature, we have a strong urge to breed!
A general counter to this is "but if a woman is already pregnant, why would you feel the need to breed with them, the point of sex has been achieved".
The easy answer is that if the point of sex is to breed, then pregnancy is like the final climax. A desire to have sex with a pregnant woman is due to this very fact, it proves they are fertile, and that in itself is sexy.

2) If someone is pregnant, it means that they have had sex.
A big pregnant belly is like a big sign saying "look, I have had sex". So, if someone has had sex, it means they might want to have sex again.
As previously said, someone can look like a complete prude, but if they are pregnant, then it means that they can't be that prudish, and that can be quite exciting to think about.

3) People are drawn to curves.
Some people like huge breasts, some people like a nice round behind, and some people like us, like a nice swollen belly.
For me I also tie this to reason number 1, a nice ripe belly feels like the ultimate reason for sex. At base level, the point of sex is pregnancy (reproduction), so it's impossible not to link pregnancy and sex together, pregnancy is like the pinnacle of the whole sex act, so why wouldn't the two mix. The fact that some people draw a line between sex, and pregnancy is surly more weird. Why would desire stop for either a woman or a man once pregnant.

I will add more points later...