Conception Dates: Accuracy of Ultrasound Estimates

Calculation of the Conception Date

Hello, I would like to ask about the first ultrasound during pregnancy.
The doctor mentioned that I am 8 weeks and 4 days along.
I would like to know what the maximum possible error in estimating the conception date via ultrasound could be, as I had relations with two men during that time.
I am quite troubled because my menstrual cycle has been irregular, and I am unsure of my last period.
If I want to find out as soon as possible who the father of the child is, what should I do? Thank you very much for your help!

Amy, 20~29 year old female. Ask Date: 2017/04/03

Dr. Huang Jianzhong reply Obstetrics and Gynecology

1.
Previously, the menstrual cycle was irregular and uncertain.
During the first ultrasound in pregnancy, the doctor stated that it was already 8 weeks and 4 days.
I would like to ask how many days the "maximum error" can be when estimating the conception date using ultrasound? Answer: Due to the irregular and uncertain menstrual cycle, it is very difficult to estimate the conception date, and the error can be quite significant.
2.
If I want to know the paternity of the child as soon as possible, what should I do? Answer: The only option is to seek a DNA paternity test; NIPT (Non-Invasive Prenatal Testing) may also be available (after 10 weeks).

Reply Date: 2017/04/03

More Info

Understanding conception dates and the accuracy of ultrasound estimates can be quite complex, especially when there are multiple factors at play, such as irregular menstrual cycles and multiple sexual partners.
When a doctor performs an ultrasound and states that a pregnancy is at a certain gestational age (for example, 8 weeks and 4 days), this estimate is based on the size of the fetus and its development. Ultrasound measurements, particularly during the first trimester, are generally considered to be quite accurate for estimating gestational age. However, there is still a margin of error. The typical range of error for early ultrasounds can be about ± 5 to 7 days. This means that if your ultrasound indicates that you are 8 weeks and 4 days pregnant, the actual conception date could be anywhere from 1 week before to 1 week after the estimated date.

In your case, if you had sexual relations with two different partners during the time frame around your estimated conception date, it complicates the situation further. Given the variability in menstrual cycles and the timing of ovulation, pinpointing the exact date of conception can be challenging. Sperm can survive in the female reproductive tract for up to five days, which means that if you had intercourse just before ovulation, it could lead to conception even if the intercourse occurred several days prior.

If you are looking to determine paternity, the most reliable method would be to conduct a DNA test. This can be done after the baby is born, or in some cases, during pregnancy through procedures such as amniocentesis or chorionic villus sampling (CVS). However, these procedures carry some risks and should be discussed thoroughly with your healthcare provider.

In summary, while ultrasound can provide a good estimate of gestational age, the inherent margin of error means that conception dates can be difficult to determine with precision, especially in cases of irregular cycles and multiple partners. If paternity is a concern, a DNA test is the most definitive way to establish the biological father of the child. It is essential to have open and honest discussions with your healthcare provider about your concerns and the best steps to take moving forward.

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