Dear Ms. Ultrasound,
I had intercourse with a someone on July 4th and July 5th. We did not use protection, and he used the pull-out method. I had intercourse with another man on July 12th, the 14th, and the 18th, and he didn't pull out at all. I had a negative pregnancy test on or around July 21st. My test was positive on July 30. An ultrasound on August 7th showed a gestational sac with a diameter of 8, and I was told I was 5 weeks GA with a conception date of July 17, give or take a day or two. An ultrasound on September 4 yielded a CRL of 26mm, and I was told I was 9 weeks, 3 days, which changed my conception date to July 14.
I did not have another ultrasound until November 6, and was measured at 18 weeks, 6 days, even though I was only 17 weeks, 6 days. I have been worried beyond measure about whose baby I am carrying. I stress to the point of not being able to eat. I need to know how accurate these were and what my chances are of the baby belonging to the second man. Please, help me with this.
I have looked over all the measurements and dates you have given me. All the ultrasounds give a conception date in between July 11 and July 16th. (Even the second trimester ultrasound!)
Please note that as the pregnancy progresses, the accuracy of the dates gets less and less. The second trimester ultrasound is not the one you would pay attention to when trying to determine paternity.
When conception dates are close, meaning within a couple weeks, it is always best to confirm paternity after the baby is born. This way you will not have to ever wonder.
With that said, it looks like the likelihood of the July 12-18th conception dates are best in your case.
-- Jane, RDMS