Pregnancy Forum - Gestational Age?

Here is my dilemma:

I found out about 1 1/2 weeks ago that I'm going to be a dad......of twins. The mother of the babies just found out as well, even though she is around 5 months pregnant.

She was on the birth control shot and the Dr.'s told her that her period would be very sporadic and absent for up to a year so she wasn't alarmed about missing her menstruation cycle. She also didn't have any morning sickness to speak of. Another important point is, she doesn't drink or smoke (thank god).

She is 20 years old and we had a short relationship that ended on good terms. We are now in a pickle.

I'll give you the exact details about her Menstruation cycle and the date we were together and hopefully you can let me know if I have reason to be concerned about her honesty.

1.) We were together on 9/2/07 Sunday and she claims this is the first day of her LMP. I do know she was on her period that day, 100% positive, although I'm not sure if it was her first day.

2.) We had unprotected sex on 9/15/07 Saturday.... (Probably right smack dab in the middle of her ovulation cycle...so based on this I'm thinking they are mine). I did not ejaculate in her. If I did impregnate her on this night the gestational age of the baby would be about 23 weeks 4 days, 165 days from her LMP. Based on a 40 week pregnancy the due date would be around June 8th.

3.) She got an ultrasound on 2/13/08 and everything looked healthy and no problems to speak of.
Here, however, is my dilemma. The report and analysis of the ultrasound reports that the big twin baby's gestational age is estimated to be around 19 weeks 4 days...and the smaller twin almost 1 week behind that and the probable due date being early July. That puts them at almost a full cycle behind where they should be.

In the research I've conducted, I realize that the accuracy of ultrasounds in determining gestational age can be sketchy at best the further along the pregnancy but they are usually only + or - two weeks.....not four weeks as in my case.

I also realize that growth rates may be different for twin babies as compared to singletons, however, that usually isn't recognized until the third trimester and twins usually grow comparable in the first two trimesters.

She swears that she wasn't with anyone else around this time. Should she be trusted and should I be concerned about the health of my babies or both. Oh.....I do plan on getting a paternity test to make sure.

Question #1- Is it possible for the ultrasound to underestimate my babies gestational age by 4 weeks?

Question #2- If they are my kids and are small for gestational age (maybe due to malnourishment) what can be done now to catch up....if anything.

Question #3- What other concerns should I have?

Thank you so much for your time.

nemt4paul