I'd definitely guess that the human part develops in the human part and then passes to the horse part somehow to develop the horse part, resulting in a 10.5 month gestation period
Biologically they would have to have intercourse with the horse parts therefore that is where the uterus is located and the centaur would get pregnant in the horse part
If I'm not mistaken, and deal with me here as my history is rusty, centaur are born by a male centaur and female human mating and the original centaur was born of one of zues' many adventures.
As I recall that was a possibility, I could likely be wrong as my interest in history tends to stray more towards real subjects and less of the legendary/fictional items.
It's possible that, with the right anatomy, a female centaur would have two vaginas and thus two uteruses: one in the front with her "human" anatomy, and one in the rear with her equine anatomy.
I've seen several artists and writers who describe male centuars with two penises, and a few even theorize that the "human" one would be able to yield only female humans, and the equine one would father only male centaurs.
Okay well if we're thinking about this logically. Then the horse part. A centaur consists of the human torso and horse body, the human chest/torso doesn't contain the uterus so, the horse part is the only option. (Doesn't matter if it's a bit human, a bit horse, this doesn't change the development of the baby as it's another 'species')
I've seen several artists and writers who describe male centuars with two penises, and a few even theorize that the "human" one would be able to yield only female humans, and the equine one would father only male centaurs.