Ultra-Sound Conception Dating

So out of the blue I got hit with, "you're going to be a dad," from a girl I hardly know. (yes, I know the whole you should have been careful speech)  she gave me the obligatory .05 seconds to think it over before asking me what I was going to do...I'm going to man up of course if the child is mine.

She sent me a ultra-sound today and said the baby is 5 weeks and 2 days old.  Which is two or three days AFTER I had sex with her for the first time.  I have reasons to believe it is not mine but am still preparing as if it is.

Let me know!

How accurate is this test?

[AnnieBrooke]

The day that ultrasound was done, the embryo would have been about 23 days old.  If an ultrasound has on it that a pregnancy is "5 weeks 2 days" along, conception in fact occurred around 3 weeks 2 days ago.  Pregnancy is dated by the first day of the woman's last menstrual period, not by the moment of conception, because the first day of a woman's period is visible and known, and the actual date of conception (which falls about two weeks later in a normal cycle) happens in a hidden place with no obvious signal.  All doctors use this dating system and so ultrasounds do too.  An embryo on an ultrasound that is titled "5 weeks 2 days" is one that has in fact been in existence only about 23 days.  Were the two of you still having sex about then?

[peekawho]

If the dates are close, you'll need a DNA test.   It can't be pinned down any closer than 3 or 4 days, plus or minus.  

[heatherlynn22]

if she had an ultrasound done 3-5 days after you had sex with her the first time and the baby is estimated to be about 5 weeks old....i'd say you're not the father. but to be 100% sure a paternity test is needed.

[AnnieBrooke]

The o.p. said the date of the pregnancy on the ultrasound (5w2d) would have been only 3 days after they met.  Not that she had an ultrasound done 3-5 days after they met.  Also, the 5w2d pregnancy dating is from the first day of her last period, as pregnancies are all dated that way, so in fact even if 5w2d is only three days after they met, his concern would be what was happening two weeks later, when she would have ovulated, not when the first day of her last period was.  For his own sake and the baby's right to know, a DNA test will be needed, but it sounds like the dates are right unless the relationship was all over and done in only one or two days.