First Trimester Dating Ultrasounds: Accuracy & Potential Errors

Question
QUESTION: Dear Brenda,

Hi! I was just wondering if you could tell me how accurate dating ultrasounds are in the first trimester? How many days at the most can they be wrong by? Can you rely on them to get an accurate date of conception? Thanks for your help:)

Laura
ANSWER: Dear Laura,

Ultrasounds done before the 12th week of pregnancy are very accurate for dating purposes. Most often on the ultrasound report that they print out for you it will say something like "6W5D +/-3D" or something similar to that. This example would mean that they are allowing a window of error of 3 days one way or the other from the date they give you. In this case - as little as 6 weeks, 2 days to as much as 7 weeks, 1 day.

But, yes, the measurements that are taken are very reliable in determining gestational age early in pregnancy.

I hope this has helped you and answered your question. I wish you well.

Brenda

---------- FOLLOW-UP ----------

QUESTION: Hi Brenda! Thanks for your last answer :) I just have a few more questions.

1. i just had my dating ultrasound and they dated my pregnancy at 6w 4d (4w 4d from conception) with a CRL of 8mm. Does this sound right to you? They also detected a heartbeat of 132bpm.

2. They didnt give me a +/- any days though on my ultrasound so how many days is the usual to be wrong by at this stage? Can you please give me an amount of days at the most they would be off by?

3. I dont understand how they are able to date a pregnancy so accuratly. Can they tell a embryos age correctly because there is such a difference week to week in size? Any info would be great!! :)

Thanks, Laura

Answer
Dear Laura,

Yes - the CRL sounds appropriate for dates. We would expect the CRL to start at approximately 5mm at Week 6 and at least double in size by the start of Week 7. And it is a good fetal heart rate.

Typically, they would not be off by any more than 3-4 days.

Yes, we can date the pregnancy that accurately because there are VERY significant differences from week to week. Measurements are significantly different and development of the fetus' different parts vary from week to week. Since we know what the norms are for fetal growth and development, we can quite accurately determine gestational age in early pregnancy. Now, as I said previously, after about the 12th week, the variations in each baby's growth pattern will affect this, so dating a pregnancy by ultrasound after that time is much less accurate.

I hope this has helped and answered your questions. Be well!

Brenda