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Dr. Andrew Rynne
MD
Dr. Andrew Rynne

Family Physician

Exp 50 years

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How To Calculate The Exact Day Of Conception?

Hi I have a paternity question. I had sex with Father A many times throughout May and June without protection and no pull out. I had sex with Father B once sometime after June 18th and he pulled out. I got pregnant and I was due February 28th, which puts my conception day sometime at the beginning of June. I was a week overdue, got induced and had the baby March 6th. Is it possible that Father B is the father? I also have extremely irregular periods but the first day of my last period was May 24th. I know my doctor probably based the due date on that date, but can having an irregular period make it wrong?
Mon, 18 Apr 2016
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General & Family Physician,  's  Response
Hi,

Thanks for posting in HCM.

I understand your concern.

The exact date of conception cannot be known precisely. For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period. Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation would have occurred.

However, it can be calculated indirectly by knowing the expected date of delivery (EDD).
EDD can be known by two means:
1. Using Naegele's rule in which we use the formula based on the first day of last menstrual period (LMP).
1st Day of LMP + 1 Year and Subtracting 3 months and then adding 7 days.
2. By means of ultrasound scan based on fetal biometry.

Pregnancy Conception Calculator can help to find out the pregnancy conception date based on the EDD. Correspondingly, it would also give the range of days during which sexual intercourse might have led to the conception, based on the factor that sperm can live inside a woman for three to five days.
You can do know it by visiting website such as:
http://www.calculator.net/pregnancy-conception-calculator.html
However, precision would only improve if your cycle was for 28 days and regular.

In case, it is inconclusive, it would be advisable to undergo DNA fingerprinting testing after birth to know who could be the biological father.

Hope the information provided would be helpful.
All the best.
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How To Calculate The Exact Day Of Conception?

Hi, Thanks for posting in HCM. I understand your concern. The exact date of conception cannot be known precisely. For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period. Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation would have occurred. However, it can be calculated indirectly by knowing the expected date of delivery (EDD). EDD can be known by two means: 1. Using Naegele s rule in which we use the formula based on the first day of last menstrual period (LMP). 1st Day of LMP + 1 Year and Subtracting 3 months and then adding 7 days. 2. By means of ultrasound scan based on fetal biometry. Pregnancy Conception Calculator can help to find out the pregnancy conception date based on the EDD. Correspondingly, it would also give the range of days during which sexual intercourse might have led to the conception, based on the factor that sperm can live inside a woman for three to five days. You can do know it by visiting website such as: http://www.calculator.net/pregnancy-conception-calculator.html However, precision would only improve if your cycle was for 28 days and regular. In case, it is inconclusive, it would be advisable to undergo DNA fingerprinting testing after birth to know who could be the biological father. Hope the information provided would be helpful. All the best.