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Q. Is it normal for the gestational age to be a week more than the last menstrual period?

Answered by
Dr. Sravanthi Nuthalapati
and medically reviewed by Dr. Sneha Kannan
This is a premium question & answer published on Apr 16, 2020

Hi, My last periods was 4 weeks 5 days back, and I discovered that I am pregnant when I checked using the home test. However when we went for a scan today the doc says the sac measures 1.75 cms and it is 5 weeks 5 days. Why is there a difference of one week when from last period which is 4 weeks 5 days to sac weeks which is 5 weeks 5 days?? Is it normal??

Dr. Sravanthi Nuthalapati

Obstetrics And Gynaecology
# Hello Momo, Welcome to Icliniq.
There is nothing to be worried about in this calculation as it's just the machine readings. you can just follow your actual dates for gestational age calculations.

As per your dates your gestational age is 4wks 5days.
You mentioned that Gestational sac diameter is 17mm, so according to the scan roughly the gestation age is around 5wks 5days.

Let me tell you Gestational sac diameter is a measurement to predict the gestational age in early pregnancy when the embryo or the fetal pole is not yet visible on Ultrasound. and there can be a deviation of + & - 1-2 weeks from the actual gestational age. So it's not that accurate.

Ideal gestational age on scan that correspond to the actual pregancy dating is the Crown rump length of the fetus. CRL is most accurate indicator of gestational age thorough out 1st 3 months if pregnancy, but it can be visualised properly post 6wks, when you can check for baby's heartbeat too.

So go for rescan after 10-14days, you will be able to check for the baby and it's measurements and also the heartbeat. And worry less, as everything is fine as of now.
Hope this answered your query. If you have any further doubts you can connect with me through chat.
Have a happy pregnancy.

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