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Accuracy of ultrasound to determine date of conception

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I would like to know the accuracy of ultrasounds in the first trimester. Would it be possible for them to be off 4-5 weeks? I slept with someone on the late night Jan 18. He used a condom, I believe I was on birth control (but cant remember for sure), and took Plan B 55 hours later. On Feb 2 I went off birth control for good & had withdrawal bleed on Feb 5-8. I had sex with another partner unprotected for the month of Feb - currently. I had an ABDOMINAL ultrasound on 4/1 that dated me 7wks, 0d with an EDD of 11/18. I had an ultrasound on 4/26 that dated me 11wks, 2d CRL 45mm with EDD 11/12. Ultrasound on 5/14 dated me 14weeks, 0days with CRL 82mm with EDD 11/12. Other ultrasounds at 19, 21, and 27 weeks all gave EDD of 11/12. Given this information, would it be possible that all of my ultrasounds were a month off and I conceived Jan 18?

Could the birth control not have worked in Jan, the embryo lived through the withdrawal bleeding, and the fetus has continued to measure small throughout the pregnancy?

When was my most likely conception date? What dates of sex could have surrounded this date?

Category: OBGYN

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Category: Infertility Specialist
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Thanks for your consult at

Totally 6 ultrasounds have all given you the same EDD - 11/12.
Based on this EDD, the last date of your period would have been 5th of February.

According to this period date, the most likely date of conception would be around 14 days after the period, that is around the 19th of February.
Ultrasound has intrinsic error of upto maximum 5 - 7 days, so the probable dates of sex would have been a week before or after the 19th of February, but in NO WAY as early as January 18th.

Abdominal ultrasound is not as reliable as vaginal scan, however ALL first trimester ultrasounds are accurate in estimating the gestational age, and have maximum error upto a week.

All of them cannot be way off by a month, so do not worry.
Also, you had a condom as protection, took Plan B, and then were on probable birth control too - so chances of conception are nil.

If the embryo had lived through the withdrawal bleeding, and then measured small in the first abdominal scan, subsequent scans at 11 weeks, 14 weeks etc would have definitely picked up the discrepancy.

You had ultrasound pretty frequently, so that possibility can be ruled out.
Please do not worry, as I strongly opine that you DID NOT conceive from the January 18th episode.

Hope this helps, please feel free to discuss further.
All the best.

Patient replied :

Thank you very much for the detailed reply. I have just been concerned because I did not take Plan B until 55 hours after the encounter. Is it still effective then? Also is it possible that the withdrawal bleeding was implantation bleeding? I read so much about that on that internet & it concerns me.

Also if for some reason I deliver early would that mean that my conception date was off and that I did possibly conceive in January?

Thanks again. I have just been worried about this all pregnancy and would like to put it behind me. I know the stress isn't good for me or the baby.


When taken within 72 hours, Plan B is still 90 % effective in preventing a pregnancy, plus condom was used.

It is very less likely that the bleeding was implantation bleeding, as it does not corelate with your period pattern ( please mention the date of the period BEFORE 5th February if you recall it ).

Also the ultrasounds are supportive of the fact that the bleeding was withdrawal bleed, not implantation bleed.

If you deliver early for some reason ( the disparity is a month ), then in that scenario, if you DID conceive from January 18th the baby would be normal, while if you conceived later, the baby would be premature (36 weeks) and a paediatrician could tell the difference.

All said and done, odds of conception from January 18th encounter are almost zero, only a DNA paternity test would conclusively put this doubt to rest.
Do not stress out, as you can already understand.

Take care.

Patient replied :

My previous period was Jan 15-18 and was on cycle with my birth control. The period I experienced in Feb was early, which I am guessing is because I stopped birth control mid pack. I stopped on the second week on Feb 1st or 2nd. I guess I am having trouble seeing that there is any chance that the Jan 18 encounter would be a possbility since all of these things are against it. Could you explain how it would be a possibility at all? Would you suggest a DNA test?


All the things are against conception from January 18th.

The ONLY possibility ( and this is like a science fiction movie, as I am just adding together improbable hypothetical situations ). The bleeding experienced on February 5th was implantation bleeding ( not withdrawal from birth control ) - Least Likely.

The Birth control, AND plan B AND condoms failed ( Least Likely )
The first scan was ONE month off the mark ( least likely ).
ALL the remaining scans continued to be one month off the mark (least likely)
This is the only situation , and it is impossible.

Yes, I would suggest a DNA paternity test if you cannot forget the thing on your own, and it is continuing to bother you to this great extent.


Patient replied :

Thank you so much for your detailed responses. Jan 18 was a huge mistake & something I have worried about all pregnancy. I appreciate you taking the time to help me. Other than for peace of mind, would you agree that DNA is not necessary to determine paternity?

Yes, apart from the absolutely irrefutable peace of mind it would offer you, DNA testing is quite unnecessary to establish the paternity, as all given facts clearly point to conception that occurred AFTER the 5th of February.

All the best.
Enjoy your pregnancy and motherhood!

Admin Message: Your consultation is already complete. You have utilized more than 3 follow-up questions already.

Dr. Aarti Vazirani
Category: Infertility Specialist
Residency: Obstetrics & Gynecolog, BJMC,Civil hospital, Ahmedabad, 2008
Post Graduate: MS (Obstetrics & Gynecology), B.J. Medical College, Ahmedabad, 2008
Medical School: MBBS, B.J. Medical College, Ahmedabad, 2004
Dr. Aarti Vazirani and 4 other Medical Specialists are ready to help you

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