Can You Date Conception From Ultrasound Data?

Jane Foley's picture


Hi Ms. Ultrasound,

I hope you can help me with my question as it would really ease my concern.

I am 31 weeks pregnant; the first day of my last menstrual period date was 26 July 26, 2009 and my usual length of menstrual cycle ranges between 28 to 30 days.

I had unprotected sex on August 6th (Day 12) and August 17th (Day 23). I really need to know whether the baby was conceived on the 6th, or the 17th.

Here are the information of my early fetal ultrasounds:

  • First Ultrasound -- September 7, 2009, with clear fetal heartbeat. Dating on ultrasound says "6 wks 1 day"
  • Second Ultrasound -- 18 September, dating on ultrasound says "7 wks 6 days"
  • Third Ultrasound -- 28 September, dating on ultrasound says "9 wks 2 days"

I did ask my OB/GYN specialist the same question that I am asking you now. All he said was it is "near impossible" that the baby was conceived on August 17th as there was a clear heartbeat during the first ultrasound on September 7th. Should the baby be conceived on August 17th (3 weeks prior to the first ultrasound), no heart beat would have been visible during the September 7th ultrasound session.

Is this true? I really need to know. I will be eternally grateful if you can help.



I agree with your Ob/Gyn. I feel the chances are very good that conception was from the August 6th encounter.

Here are the possibilities from your Last Menstrual period and your three ultrasounds.

  • LMP July 26 -- Average day of conception August 9
  • Ultrasound #1 Sept 7th: 6w 1d -- Average day of conception August 9
  • Ultrasound #2 Sept 18th: 7w 6d -- Average day of conception August 8
  • Ultrasound #3 Sept 28th: 9w 2d -- Average day of conception Aug 8

These calculations assume a woman has a 28 day cycle and ovulates on day 14.

I always advise a paternity test in the end, because any question resting in the back of your mind will haunt you forever. It's better to be sure.

With Regards,