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Forum Name: Obstetrics

Question: How accurate is an ultrasound conception date?

 lefi1985 - Wed Jul 14, 2010 10:37 am

I had sex with Partner #1 on 2/15/10. Period on 2/25/10. Period on 3/26/10. Sex with Partner #2 on 4/3/10 and 4/8/10. Today I am 15 weeks 3 days pregnant, my due date is January 1, 2011. Do I need to get a paternity test? I had an ultrasound on 5/13/10 saying I was 6 weeks 4 days pregnant, the doctor said I had conceived 4-5 weeks prior, which would be Partner #2. How accurate is that?
 manyquestions - Wed Jul 21, 2010 7:01 am

I have a similar question regarding accuracy of conception date determined by ultrasound. According to my ultrasound dating, conception is about a week later than I expected. My husband and I had sex on 5/23 (had a positive OPK test that weekend as well) so I figure conception was right around that time.

According to my u/s conception was on 5/30. The only other activity that occurred between that time was some manual stimulation of me by my partner, no ejaculation, and highly unlikely that there was even any fluid on his fingers at the time he touched me.

Is it more likely that my conception date is miscalculated by the u/s (done at 9 weeks) or that pregnancy occurred from the manual stimulation time?
 Debbie Miller, RN - Wed Jul 21, 2010 12:12 pm

User avatar Hello,
That is very accurate and unless you also had sex with partner # 1 during the month of April, # 2 is the partner. A paternity test would only be necessary if you want it or need it for recovery of child support. Since you had a period in March, the February sexual encounter did not result in pregnancy.

Best wishes.


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