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Had sex and was on birth control. Done with ultrasound test. Is paternity test required to determine pregnancy?

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Practicing since : 2001
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I have a question about the accuracy of ultrasound for conception dates.

i had sex with Partner A once (huge mistake) in the late night XXXXXXX 18/early morning XXXXXXX 19. was on birth control (YAZ) consistently for a month prior, used a condom, and took the morning after pill within 50-55 hours. XXXXXXX 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 XXXXXXX - 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 XXXXXXX 19 encounter. Is a paternity test necessary to determine this with certainty? If so, is there a way to get a noninvasive prenatal paternity test before the baby comes? I am just nervous about the accuracy of an early ultrasound performed abdominally.

Also would the morning after pill still have been effective if taken 50-55 hours after the encounter?
Posted Thu, 5 Sep 2013 in Pregnancy
Answered by Dr. Rakhi Tayal 1 hour later
Brief Answer:
You are not pregnant form XXXXXXX 19 encounter.

Detailed Answer:
Thanks for trusting us with your health concern.
As per all of your ultrasound scan reports, your ovulation was between 15th-20th February. The chances of conception due to the encounter on 19th XXXXXXX are ruled out completely.
Paternity test is not required at all as the abdominal ultrasound shows a discrepancy of only one week and not a month.
There is no prenatal paternity confirmatory test that is non invasive and does not pose a risk for your baby.
The morning after pill shows an efficacy of 95-97% if taken upto 72 hours after the intercourse. So, there is nothing to worry.
Please feel free to discuss further. I will be glad to answer the follow up queries that you have.
Please accept my answer in case you do not have further queries.
Wishing you good health.
Dr. Rakhi Tayal.
Above answer was peer-reviewed by
Follow-up: Had sex and was on birth control. Done with ultrasound test. Is paternity test required to determine pregnancy? 29 minutes later
I believe all of my ultrasounds were dated using abdominal ultrasound. What is the margin of error for abdominal ultrasounds in the first trimester?

I have read that you cannot determine paternity from ultrasound, why is it so definitive in this case?

Also what is the earliest day of intercourse that could have contributed to this pregnancy?
Answered by Dr. Rakhi Tayal 23 hours later
Brief Answer:
The error is +/- 1week 3 days

Detailed Answer:
Thanks for writing again.
The margin of error in abdominal ultrasound during first trimester is upto 10 days.
But since repeated scans are giving the same EDD, the chances of error are very less.
Paternity cannot be detected from an ultrasound but the time of ovulation can guide regarding the time of conception.
The earliest day of intercourse is near 13th February.
Hope my answer is helpful.
Do accept my answer in case there are no further queries.
Above answer was peer-reviewed by
Follow-up: Had sex and was on birth control. Done with ultrasound test. Is paternity test required to determine pregnancy? 24 minutes later
So even though paternity cannot be determined from ultrasounds, we can definitely rule out XXXXXXX 19 in this case because of the time of conception? Is this 100%? There is no way XXXXXXX 19 could have contributed to this preg?
Answered by Dr. Rakhi Tayal 9 minutes later
Brief Answer:
Yes. XXXXXXX 19 can be ruled out.

Detailed Answer:
If the conception would have happened on XXXXXXX 19, then your EDD would have been 12th October. All the ultrasound scans are suggestive of conception near the middle of February. So, there is no need to worry.
Above answer was peer-reviewed by
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