Maiesiophilia, also spelled maieusiophilia, is the sexual attraction to pregnant women, or women who appear to be pregnant. Although, strictly, maiesiophilia doesn't have to include a sexual attraction to birth, it frequently does. Many people assume that this paraphilia is related to pedophilia, but generally speaking, it isn't. Most maiesiophiles are primarily, if not solely, attracted to women who are fully sexually mature, as only they are capable of being pregnant.
The actual foci of attraction are multiple. The rounded belly is the most common by far, but isn't the only focus. Others include the swelling of the breasts, the movement of the babies inside, and sometimes just the very notion. I have personally known a few maiesiophiles who were extremely aroused by small, barely visible figure changes, or by those very few women who go through their entire pregnancy without showing to any significant degree. A few are even aroused by the birth itself.
A slight variant on this paraphilia exists as well, where the attraction is to being pregnant. This is sometimes referred to as gravidophilia. Both paraphilias appear to be fairly common, or at least well represented on the internet, and are also markedly common in the furry fandom. In fact, on FurNet, there are a number of channels dedicated to it. At various times, #WISWRP, #pregfur, #metahysteria and #maiesiophilia have been hotbeds of activity. Other channels cater to the related fetishes of inflatophilia, unbirth, oviposition and lactation.
References: Personal experience, interviews with other maiesiophiles, chiefly on FurNet, and Wikipedia's limited treatment of the matter. For a fetish that seems to have so many sites devoted to it (Google pregnant + porn some time!), there's not a lot written about it.