Doc C and Faldage have it lettered correctly, at least according to my present understanding.

When I was an extraordinarily naive first-year medical student buying my first stethoscope from Malin's Ethical Pharmacy on Fourteenth Street (south side) between First and Second Avenue, I saw on the shelf behind me a display of tubes of PreMarIn Vaginal Cream. Blushed quietly to myself and decided it must be a contraceptive cream and that the name had to come from "Pre-Marital Intercourse". Mused for a moment about how bold manufacturers were getting (this is the Sixties, remember, the earliest days of the Sexual Revolution)...

It wasn't until years later that I put together the generic name ("conjugated equine estrogens") and the source - pregnant mares' urine - and smiled, and blushed quietly to myself again at my innocence.

And it wasn't until more years had passed that it occurred to me that perhaps I had no real reason to accept the second explanation as having any greater validity than my original one...(except, of course, the Premarin isn't a contraceptive)

Just to be provocative: is that _really_ where the name comes from? Has it been "verified by competent authority," as Pooh-Bah said? As I realized, just because something makes sense doesn't necessarily mean it's correct! :-)

EDIT: For another look at that last statement, read "The Nine Mile Walk" by Harry Kemelman.
For a very abbreviated review, try
http://www.geocities.com/Athens/8462/ninemile.htm