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How to determine the date of conception?

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Posted on Tue, 26 May 2015
Question: Hello, I found out I was pregnant but have had two possible partners.

I came off the pill and had my period on the 26/01/15, my first day of next period after being off the pill was the 01/03/15. I had sex firstly on the 22/03/15 and then with the second man on the 31/03/15 but this time I took the morning after pill. I am now (according to the intervaginal dating scan) 8 weeks pregnant exactly (today 04/05/15). Is there any chance that the baby belongs to the 31/03/15 encounter?
doctor
Answered by Dr. Deepti Verma (45 minutes later)
Brief Answer:
Difficult to predict

Detailed Answer:
Hi,

I have gone through your question and understand your concerns. According to the calculation of gestational age by last menstrual period( 1/3/2015) it is 9 weeks 1 day. Since the gestation age by ultrasound is 8 weeks, that may be due to delayed ovulation and hence delayed conception.

Both the dates you specified therefore seems to fall after ovulation. Hence it is extremely difficult to predict which caused pregnancy. The pregnancy can be due to unprotected intercourse on 23/3/15 or 31/3/15, as both can be after ovulation, and the exact ovulation date cannot be determined retrospectively.

Moreover, morning after pill does not have a 100% success rate in preventing pregnancy. Since the gestation age is calculated by last menstrual period and not the date of ovulation or fertilization, in your case it is not possible to predict the date of conceiving, more so if both dates are after suspected ovulation.

If you are very worried, I can only recommend you get a DNA testing of the fetus by chorionic villus sampling at 11-13 weeks and confirm the paternity.

Hope you found the answer helpful. Please do get back for further queries.
Wishing you good health.
Regards,
Dr Deepti Verma
Above answer was peer-reviewed by : Dr. Prasad
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Follow up: Dr. Deepti Verma (1 hour later)
Hello - thank you for your quick response.

If the morning after pill has a 95% success rate and the 31/3/15 being almost a month after my first period, would it be your opinion that the conception date would more likely be then 22/3/15 as opposed to the 31/3/15?
doctor
Answered by Dr. Deepti Verma (19 minutes later)
Brief Answer:
Exact date cannot be determined

Detailed Answer:
Hi, thanks for responding.

You are right in your interpretation. However based on the details provide, I am unable to localise the, exact date of conception since ovulation is unpredictable in this case.

Wishing you good health.
Dr Deepti Verma
Above answer was peer-reviewed by : Dr. Prasad
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Answered by
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Dr. Deepti Verma

OBGYN, Maternal and Fetal Medicine

Practicing since :2009

Answered : 5068 Questions

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How to determine the date of conception?

Brief Answer: Difficult to predict Detailed Answer: Hi, I have gone through your question and understand your concerns. According to the calculation of gestational age by last menstrual period( 1/3/2015) it is 9 weeks 1 day. Since the gestation age by ultrasound is 8 weeks, that may be due to delayed ovulation and hence delayed conception. Both the dates you specified therefore seems to fall after ovulation. Hence it is extremely difficult to predict which caused pregnancy. The pregnancy can be due to unprotected intercourse on 23/3/15 or 31/3/15, as both can be after ovulation, and the exact ovulation date cannot be determined retrospectively. Moreover, morning after pill does not have a 100% success rate in preventing pregnancy. Since the gestation age is calculated by last menstrual period and not the date of ovulation or fertilization, in your case it is not possible to predict the date of conceiving, more so if both dates are after suspected ovulation. If you are very worried, I can only recommend you get a DNA testing of the fetus by chorionic villus sampling at 11-13 weeks and confirm the paternity. Hope you found the answer helpful. Please do get back for further queries. Wishing you good health. Regards, Dr Deepti Verma