Question(I have a clarification question re: this answer I found on the website. The expert indicates that based on LMP 10/1 and avg 33 day cycle, conception is 10/21 and thus is 10 weeks as of 12/17 date of answer. The ultrasound found her 8 weeks 1 day on Nov. 30th. From Nov. 30 to 12/17 would be the passage of two weeks plus, not be ten weeks? if the "but..." part of her answer is correct, you would expect the patient to be 9 weeks at November 30 and 11 weeks as of December 17, 2007 Your clarification would be appreciated)
Expert: Brenda - 12/17/2007
Question
I was with a girl in the early morning hours of Oct. 28th, 2007 (4am). I later found out that a friend of mine was with her on both the 20th and 21st of October 2007. I did not finish inside her and my friend does not remember if he did. All encounters were unprotected, I know how stupid it was, no need to reiterate.
The girl called me in mid-November to tell me she was pregnant. At first I did not believe her, but I went to the doctor with her on November 30th. Via an internal ultrasound I know that she is definitely pregnant.
Her last LMP was October 1st and she told the doctor that her average cycle is 32-34 days, but her cycle is erratic. Her doctor set the due date for July 19th and advised her that she is 8 weeks and 1 day.
How accurate are ultrasounds in determining due dates?
Can a woman ovulate that late in her cycle?
I know that the only definitive answer to paternity will be with a DNA test. However, can you offer any opinions/similar stories/advice to share? Thanks for your help.
Answer
Dear Keith,
A woman typically ovulates 14 days prior to the beginning of her next period. So, if she typically has average 32-34 day cycles, we would expect her to ovulate on approximately cycle day 18-20.
If her LMP was 10/1, and using an average 33 day cycle length, I calculate an estimated conception date of 10/21. This would make her 10 weeks today with a due date of 7/13/08. BUT, if ultrasound has measured the fetus at one week younger and given a due date of 7/19/08, that would indicate that conception took place approximately 10/27-28.
Ultrasounds are actually extremely accurate in dating a pregnancy when done in the first trimester (12 weeks and before). After the 12th week (and some say 20th week), babies can grow at different rates based on several factors, so it can lessen the accuracy just slightly.
Unfortunately, if you are suspecting that the baby is not yours, you will have to wait until after delivery and request a paternity test. Otherwise, at this point, based on the ultrasound measurement you've shared, it does indicate a later conception date in this instance.
I hope this has helped you and answered your question. I wish you well and a Merry Christmas.
Brenda
AnswerI'm having a hard time understanding the question, but I can see a discrepancy in the previous answer given. When periods are not regular, it's best to just rely on the ultrasound. So if the ultrasound says she's 8 weeks 1 day on Nov. 30, by my pregnancy calendar that gives a due date of July 11, and a conception date of Oct. 19. These dates are not precise, but an ultrasound at 8 weeks will not be off by more than 7 days (usually less). I hope that answers your question.
