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How to determine paternity based on conception?

Mar 2016
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Practicing since : 2011
Answered : 4045 Questions
My daughter is trying to determine paternity based on conception. Her last menstral period was december 6, 2012. She is on the pill. She has regular periods based on this. She had sex on Dec. 13, 27 and XXXXXXX 1. She has had two ultra sounds, this one today said based on LMP 7weeks 4 days, yet they say she conceived around Dec. 30th. Can you please define how she can be 7 weeks 4 days on an ultra sound yet only conceived on the 30th give or take a few days?
Posted Sun, 24 Feb 2013 in Women's Health
Answered by Dr. S Patra 1 hour later

Thanks for posting your query. I have gone through your history in detail.

I hope that following information may be helpful to your daughter in case of disputed paternity.

According to last menstrual period (LMP), her expected due date was on 6th January as previous cycle was regular. So she ovulated around 23rd December and her fertile days was from 20th to 26th December. As she involved in unprotected sex on 27th December, close to fertile days, then chance of being pregnant was there.

By ultrasound, if she conceived on 30th December, then calculated LMP was around 14th-16th December which is 7-10days lagging behind the original LMP. Here, gestational age by ultrasound is 6 week plus on today.

Basically, ultrasound gives gross & detailed view on the basis of fetal maturity at the time of scan. But it is also known that development of fetus or embryo may be advance or lagging as in this case. Therefore, fetal maturity may not always correspond with actual gestational age as calculated from original LMP.

In that respect, if the gestational age by ultrasound is within one week of the mother's original LMP date, then we think it is within normal limit. Further maturity of fetus can be lagging behind the period of gestation in case of early week of pregnancy (developing embryo) or late scan (small for date).

Therefore, presuming her original LMP, she conceived in the month of December and possibility of being father is high enough for her current partner.

If you still have doubts then you need to perform the DNA test which is superior to all investigation and gives the most reliable solution.

Hope I have answered your query and helped in determining probable time of conception. If you have any further questions I will be happy to help.

Wish good health for your family.

Dr Soumen.
Above answer was peer-reviewed by
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