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Midwives/Paternity - Ultrasound Accuracy


QUESTION: I have a question about the accuracy of ultrasound for conception dates. Sorry for the length.

i had sex with Partner A once in the late night Jan 18/early morning Jan 19.  was on birth control (YAZ) consistently for a month prior, used a condom, and took the morning after pill within 50-55 hours. Jan 18 was also the last day of my period according to my birth control pack, but I cannot remember if I actually had a period.

I was with Partner B end of Jan - present. I had what I believe to be a LMP on Feb 5. I went off birth control at this time & started taking prenatal vitamins because we decided we wanted to try for a baby. On March 6 I found out I was pregnant by a FAINT line on a hpt. The same day I went to the OB, and my urine test came up negative. They gave me a blood test & it was positive.

My first ultrasound was on 4/1. It was performed ABDOMINALLY, therefore I am not sure of its accuracy. It stated that was 7w0d with an EDD of 11/18. I had another ultrasound on 4/26 (abdominally) stating 11w,3d EDD 11/12 CRL 45mm. Another ultrasound on 5/14 dating 14w, 0d dating 14w,1d EDD 11/11 CRL 85mm. Ultrasound on 6/18 dated 19w, 0d EDD 11/12. Ultrasound 7/5 dated 21w, 2d EDD 11/13. Last ultrasound on 8/12 dating 26w,6d EDD 11/12.

My question is is possible that I got pregnant from the Jan 19 encounter?

ANSWER: Dear Grace,
If a urine test came back negative and a blood test came back positive, that can be a strong indication that you were just barely pregnant, maybe 2-3 weeks after conception, not more.

Based on the 4/1 ultrasound and the EDD of November 18, your date of conception approximately February 25. The 4/26 ultrasound gives you an estimated date of conception of February 20th. I would say that your EDD of 11/12 is likely the most accurate one, if you factor in your actual LMP of Feb 5. This means you conceived on or around February 20th.
Since all babies grow at the same rate in the first trimester, give or take a day or two, early ultrasounds in the first trimester are the most accurate. After about 12 weeks they can be off by as much as two weeks, since growth and development can vary more widely at that point. I would be reassured by those first two ultrasounds. They couldn't be off by the 4 whole weeks you would need in order for the conception to have happened in January.

Based on the dates and ultrasound results that you listed, I think you can be sure that you conceived in late February.

Best of luck,

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QUESTION: Thank you so much! I have just worried myself sick with the possibility that Jan 19 contributed to this pregnancy. So even though my 11 week US was abdominal, it will still have an accuracy of +/- a week? I have just read online that you cannot determine paternity from ultrasounds, therefore I just wanted to make sure!

You're right that you can't determine paternity from ultrasounds, but that's only when both men have an equal likelihood of being the father, meaning, the woman had sex with both men during the time that the ultrasound says she conceived. Since one of them was with you about a month before your ultrasound says you conceived, he can't be the father. If someone wants 100% proof, do the paternity test. But the dates don't add up for the January fellow, only the February one.  


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