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Prenatal paternity testing

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My husband and I are occasional swingers (we usually do this once or twice a year). On September 14th, we met with another man. I have been off birth control for over a year because of some issues I was having with it. Because of that, both my husband and the other man wore condoms. A few weeks later, we found out I was pregnant. At the first ultrasound (7 weeks), my doctor used measurements to determine a contraception date of 9-4 or 9-5. How accurate is this? We are a little anxious, obviously, as my husband always uses condoms as well. We're considering prenatal DNA testing to put our minds at ease, but if the conception date is accurate, then we would not need that. Thank you.

Category: OBGYN

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Category: Infertility Specialist
 22 Doctors Online

Thank you for your consult at I can understand your concern.

What you basic question boils down to is this - how accurate is a first trimester ultrasound scan ?

The answer is - First trimester scans are accurate within a limit of + / - 5 to 7 days.

Would you provide me with the following details please -
1. Your last menstrual period
2. Your normal menstrual pattern - length and duration of menstrual cycles.
3. Date of ultrasound scan and exact gestational age of baby in weeks and days found on that scan.

As the date of conception provided ( 4th or 5th of September ) is relatively close to 14th September, considering the margin of error of ultrasounds, I would definitely recommend prenatal DNA testing to put your mind at ease.
It is infinitely more accurate.

All the best.
Hope this helps, Please feel free to ask for further clarifications.

Patient replied :

2.duration of past cycles: 28,31,25,28,27,19
3. 10/21/13 8 weeks 3 days

Based on the dates you have provided, it is apparent that you conceived sometime around the first week of September, not later than 10th September, I would say, based on your cycle length.

Again, this puts the margin of error at 4 days only , I would advice a DNA test to be sure.

Presumptively speaking, the date of conception would not be as late as September 14th , the odds of your having conceived from that encounter are very low, just have the test to be 100 % sure.

Take care.

Patient replied :

Thank you Doctor.

You are welcome. Best wishes.

Dr. Aarti Vazirani
Category: Infertility Specialist
Residency: Obstetrics & Gynecolog, BJMC,Civil hospital, Ahmedabad, 2008
Post Graduate: MS (Obstetrics & Gynecology), B.J. Medical College, Ahmedabad, 2008
Medical School: MBBS, B.J. Medical College, Ahmedabad, 2004
Dr. Aarti Vazirani and 4 other Medical Specialists are ready to help you

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