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How Accurate Are Dating Scans At 9 Weeks?

Posted on Thu, 30 Aug 2012

Question: Hi, could you please tell me how accurate are dating scans at 9 weeks? If im unsure how long my cycle is would the scan pick up time of ovulation according to crl measurement? For eg: if my last lmp was on 8 july but according to the scan it points to 11 july,does that mean that I would have ovulated 3 days later?? Meaning instead of ov on day 14 I would have ov on or around day 17/18 of my cycle?

Answered by Dr. Mahesh Koregol (3 days later)

Hi,

Thanks for your query.

I read your query and I understand your concerns. Following is reply to your query:

1) Dating scan is accurate but has error of one week variation (plus or minus one week).

2) the variation by 3 to 4 days is common by CRL measurement.

3) Hence it cannot be predicted that your ovulation was delayed to around 17 18 th day. Your ovulation could have been accurately on 14 th day only.

4) This scan cannot predict accuracy by days. 3 to 4 days error is acceptable.

I hope I answered your query. I will be available for any followup queries you have.

Regards,
Dr.Mahesh Koregol
IVF & Infertility Specialist.

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

Follow up: Dr. Mahesh Koregol (2 hours later)

ok, not sure if i understand. so for example if someone has a very long cycle eg:36 days where ovulation would most likely happen around day 22, so wouldnt ultrasound scan pick that up? for example if LMP was 8 Feb and EDD is 22nd Nov would that mean that ultrasound picked up that cycle was around 36 days long and conception took place later in the cycle? becouse in that case there is a 8 day variation (instead of normal 28 day cycle, cycle is 36 days long) ? is this correct?? basically my question is if ultrasounds would pick up late conception, obviously at early stages of pregnancy crl measurements would be different between a 8 week and a 9 week fetus, correct??

Answered by Dr. Mahesh Koregol (12 minutes later)

Hi,

Thanks for reverting back.

You are right. In 36 days cycle ovulation is around 22nd day. But if you do ultrasound of that fetus conceived on 8th or 9 th week it will definitely show the growth of fetus corresponding to the gestation. But still 3 to 4 days margin of error is expected.

This is because position of the baby at the time of scan and the shadow by adjoining structures. The EDD has to be calculated by LMP and ultrasound EDD should not be considered as it will not be accurate.

If ultrasound is done in third trimester ( after 28 weeks), you may find the difference of 2 weeks between period of gestation and ultrasound !!

Hence I suggest you take your EDD date based on LMP date.

If most accurate dating is required, it is better to do ultrasound by 6 weeks.

I hope I answered your query. I will be available for any followup queries you have.

Regards,

Dr.Mahesh Koregol
IVF & Infertility Specialist.

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