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Q. The gestational age in scan is different from the weeks when calculated from my LMP. Why?

Answered by
Dr. Eeshani Dutta
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Feb 19, 2019

Hello doctor,

I am 27 years old and pregnant. My last period was two months back but the baby only shows six weeks and three days gestation as of today with a heart rate of 77. I have a picture of my ultrasound from today and I was hoping you could explain what it was showing.

#

Hi,

Welcome to icliniq.com.

For a lady, with regular periods previously, generally ovulation occurs around 14th day of the cycle, so fertilization can take place only after that. In your case (if your periods are regular), you ovulated around 22nd two months back and fertilization took place only after that. So originally, the baby will always be two weeks less than what is calculated by LMP.

Secondly, first-trimester USG can have an error of up to seven days while second and third trimester up to 2 and 3 weeks respectively, so do not worry if the calculation and USG are not exactly the same.

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