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How to determine my conception date?

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Practicing since : 1999
Answered : 1586 Questions
Hi! I was diagnosed with PCOS and have irregular periods for as long as I can remember. A year ago my period became normal 30-35 days. I found out I was pregnant and had an ultrasound last August 5, 2013. Based on my LMP my AOG was 7 wks 2 days but based on CRL in the ultrasound, I was only 6 wks 3 days. My question is: What date did I conceive?
Posted Fri, 31 Jan 2014 in Women's Health
Answered by Dr. Timothy Raichle 11 minutes later
Brief Answer: Hello, I would be happy to help... Detailed Answer: Given the irregular cycles and discrepancy between the LMP dating and the ultrasound at this early gestational age, I would date your pregnancy based on the ultrasound. 1. You were 6 3/7 wks on 8/5 2. This would give you a due date of 3/28/2014 3. Conception occurred on 7/5/2013 (give or take a few days) I hope that this helps - let me know!
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Follow-up: How to determine my conception date? 6 minutes later
What does date of conception mean? Is this the time I had intercourse? What does "give or take a few days" mean? The reason I am asking (and I am very ashamed and worried about this), I had intercourse with two men. The first was XXXXXXX 30, July 1, July 6, and the other July 4. Can I determine the father based on the results of my ultrasound? Thank you.
Answered by Dr. Timothy Raichle 4 minutes later
Brief Answer: Thank you for the followup... Detailed Answer: Date of conception = about when you got pregnant / had intercourse that led to pregnancy. Sperm can live in the cervix for up to 72 hours, so even if you ovulate on a specific day, intercourse 72 hours before or after can lead to pregnancy. In other words, there is a larger window within which to get pregnant than you think. You cannot determine who the father was until after the pregnancy is over. Intercourse on 7/4 and 7/6 could both easily have produce this pregnancy. I know that this is an unfortunate situation, but it cannot be fully worked out until the baby is born. I am sorry. Please ask if you have any followup questions.
Above answer was peer-reviewed by
Follow-up: How to determine my conception date? 12 minutes later
Thank you for your answer. Is the 7/5 conception date accurate or is there room for discrepancy? I've been online for so long already and read different opinions. Some say there's a discrepancy of +/- 5 to 7 days from date of conception. Is this true or is this an outdated opinion already? Are online calculators accurate enough? To clarify, when the ultrasound result says AOG by 6 3/7 weeks, that is not the date of conception? Why is the date of conception 2 weeks apart from the given AOG. I guess the main question I really want answered is if the ultrasound result is definitive enough to determine date of conception or there is room for error? If yes, how much? Thank you for your sympathy. I really want it to be the first guy, because the second one was just a one night stand. Thank you.
Answered by Dr. Timothy Raichle 1 hour later
Brief Answer: Here are the facts... Detailed Answer: Given your cycle duration and variation, there is probably about a 3-5 day window within which you ovulated. Also, as I stated, sperm can hang out for about 2-3 dyas. Third, there is about a +/- 5 day window of error in ultrasound at 7 weeks. This all adds up to some significant uncertainty regarding the father of this baby. AOG = age of gestation. The weeks gestation is measured from the first day of your last period. When this is uncertain, ultrasound is more accurate. So, as I stated, based on the ultrasound which is the most accurate thing to go on based on your history, the most likely date of conception was 7/5 +/- 2-3 days based on the length of time sperm can live in the cervix and still fertilize an egg. I hope that this clarifies the situation.
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