How Is The Gestational Age Of A Fetus Calculated?

Posted on Fri, 18 Sep 2015

Medically reviewed by Ask A Doctor - 24x7 Medical Review Team

Fri, 18 Sep 2015 Answered on

Fri, 9 Oct 2015 Last reviewed on

Question : Yesterday September 17 I was given a vaginal ultrasound which showed my baby to be 6 weeks old. We could see a heartbeat. I am confused about how people date weeks in pregnancy. If I had sex on August 28th (3 weeks ago) is there any way my ultrasound could show a 6 week old fetus?

Answered by Dr. Mahesh Koregol (17 minutes later)

Brief Answer:
LMP is tha basis

Detailed Answer:
Hi,

I read your query and I understand your concerns.

Period of gestation is calculated based upon last menstrual period and not date of sex. Please calculate based on date of last period.

On ultrasound scan, pregnancy dating is done by measuring CRL on ultrasound scan. This is accurate at this stage of pregnancy.

Let me know if you need any more clarification.

Regards

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Mahesh Koregol (26 minutes later)

Can you tell from the ultrasound I included if my doctor meant the baby was 6 weeks gestational or 6 weeks fetal age? I'm pretty sure she meant fetal age because my period was not regular and I had spotting a one day of a period at the beginning of this month. She estimated that I conceived on 8/8 give or take 3 days.

Answered by Dr. Mahesh Koregol (3 minutes later)

Brief Answer:
Period of gestation

Detailed Answer:
Hi,

This appears to be period of gestation of 6 weeks from last period date. This is done by calculating CRL on ultrasound scan.

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Mahesh Koregol (18 minutes later)

What does CRL stand for, and if this is the case then that means that I could have conceived 3 weeks ago?

Answered by Dr. Mahesh Koregol (5 hours later)

Brief Answer:
3 weeks ago you have conceived

Detailed Answer:
Hi,

CRL stands for crown rump length ( head to bottom of fetus. This is measured in early pregnancy to predict growth and period of gestation.

You are likely to have conceived approximately around 3 weeks back. It is difficult to say when exactly.

Let me know if you need any more help.

Regards

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar