The first day of my last period was December 28th.

If your period was December 28th, and you usually have a 28 day cycle, than you would have conceived around January 11th, 2010, and will be due around October 4th, 2010. If they are basing 9 weeks and 4 days off of an ultrasound, than it is probably accurate, meaning you would have ovulated very early in your cycle... around January 1st. Also, keep in mind that sperm can live up to 3 days inside of you, so it's not necessarily whomever you slept with on the day you supposedly conceived, but also the 3-4 days prior to that up to a couple days after you ovulate. It sucks that you don't know who the father is, and I don't know anything about paternity tests so I can't help you there... but I would like to remind you that having unprotected sex with numerous partners does increase your risk for std and infections... which is especially important now that you are pregnant. Congratulations on your baby!

Technically you're not pregnant for the first two weeks of your pregnancy. I don't completely understand it but you concieve two weeks after the 1st day of your last period and your doctor figures out your due date by that date. As apposed to asking you the dates that you had sex...cuz there really isn't any way to tell when you concieved by when you had sex. My friend had a paternity test when she was pregnant, if I remember correctly it took a month or so to get the results. You do have to have blood from one of the guys though.