Can Ultrasound Date Precisely?

Jane Foley's picture


Dear Ms Ultrasound,
I am 34 years old and this is my first pregnancy. The first day of my last period was 26 July, usually a cycle of 32 days. Conception date is either 8 or 10 July .

I went to the doctor and had a first early ultrasound scan yesterday on 23 August. I was supposed to be 8 weeks and 3 days. But, the scan shows 9 weeks and the due date is computed as 28 March. All my previous calculations pointed to a due day of 2 April.

Please kindly explain if the ultrasound scan can pick up differences in 3-5 days of gestational age.

Thank you!


No, it is not possible to differentiate a 3-4 day difference. The dating process is not an exact science. I may have measured your pregnancy at a different point. Just 1 millimeter can change the dates. Also, just because intercourse happened on a certain day does not mean conception happened on that day. Sperm can live up to 72 hours and conception can happen days later.

My thoughts for women who are worrying unnecessarily are to try and connect with the life that you have created. Don't cause yourself anxiety over a few days. Enjoy the pregnancy! It's an amazing time in a woman's life.

-- Jane, RDMS