I believe its just the pregnancy hormone itself that makes you that way. Its also psychological i.e. I'm already pregnant so I can just enjoy it. My ex-wife was the same way. I wish they bottled it too. Maybe she wouldn't have started hating sex down the road after her hysterectomy . . .
LOL! I'm not sure if there is a medical term or specific hormone that explains this. For me, it varied while I was pregnant with each child. With my last one though, I was extremely "in need." It was crazy for me! I just figured it was because my body was changing and everything was so super sensitive--especially my sexual desire. The way some women get morning sickness or can't eat or smell certain things is comparable to the cranked up sexual desire. At least that's how I see it. When I spoke to other pregnant women about it, some agreed and some almost cried mentioning the word, lol.
It would be a nifty thing to have around when needed though. I never felt so aroused, animalistic, sexy, sexually aggressive as I did when I was 20lbs over weight with feet like hams :p