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Q. How accurate are ultrasounds in determinig the gestational age and conception?

Answered by
Dr. Ushasi Mukherjee
and medically reviewed by Dr. Vinodhini. J
This is a premium question & answer published on May 22, 2020

Hello doctor,

How accurate are early ultrasounds at determining gestational age and conception? Based on my 6th-week scan I am 23 weeks today means I conceived around December 25th. Is it possible that I actually conceived around December 5th and baby is measuring small? (I have had six ultrasounds and the baby has never measured more than 2 days off from the date given at my 6-week scan) even though my last period last came on November 28th she has never measured on track for that.

Dr. Ushasi Mukherjee

Obstetrics And Gynaecology


Welcome to

The first-trimester ultrasound scans done before 12 weeks are the most accurate ones in the assessment of gestational age with an error of only + /- 5 to 7 days in the assessment of gestational age. So they are used for obstetric dating and determination of expected date of delivery. Moreover, if you have done 6 ultrasounds and the date calculated from each of them corroborates and does not lag behind then probably your baby is growing well and it is not small as you apprehend.

The date we calculate from the first day of last periods is accurate only if the periods were regular for the last 3 months and there was no history of miscarriage or ovulatory disorder like PCOS (polycystic ovary syndrome) or contraceptive pill intake. In case of any discrepancy the date from dating scan at 6 weeks are accurate and to be considered. It is a probability that you actually conceived in later part of the cycle which leads to this discrepancy between the date and USG.

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