Definition – What is Pregnancy Fetish? What are the sub fetishes to it and the difference with other similar kinks?
Wikipedia defines Pregnancy fetish (also known as maiesiophilia or maieusophoria) as a context where pregnancy is seen by individuals or cultures as an erotic phenomenon. It may involve sexual attraction to women who are pregnant or appear pregnant, attraction to lactation, or attraction to particular stages of pregnancy such as impregnation or childbirth. 
This should not be confused with Breeding fetishism. For more please read: Breeding (Impregnation)
Amongst those who have the fetish, there are many subtle distinctions and sub-fetishes, so not all are the same. In some cases, it coincides with an affectation for lactation and can be connected to inflation fetishism. It is not, however, at all connected to pedophilia (despite the apparently common assumption). 
The main attraction for many pregnancy fetishists is the curve of the belly leading into the pubic area. For some the bigger the belly becomes the more aroused they feel. The breasts also increase in size causing a sub-class of pregnancy fetishism that leads to breast expansion. 
Many people who have this fetish have a limit to how large the belly or breasts can become before they are no longer aroused. Some are more aroused by the breasts and belly increasing far beyond natural limitations. This is when a pregnancy fetish can become an inflation fetish. Others are aroused by the thought of impregnation and the inevitable pregnancy. This has more to do with potency than the actual pregnancy fetish. 
Many women, as well as men, share this fetish. Some women feel sexier, and more desirable while pregnant. Towards this end, there are lingerie lines made exclusively for pregnant women as well as special sexual poses that are low impact.
Some women might even orgasm longer while pregnant. This can be from an increase in pressure on the g-plate or even on the cervix. It should also be noted that many couples have sex while pregnant, even if neither partner has a pregnancy fetish. 
 Wikipedia https://en.wikipedia.org/wiki/Pregnancy_fetishism
 Psychology Wiki https://psychology.wikia.org/wiki/Pregnancy_fetishism