Accuracy of Ultrasound

QUESTION

Dear Ms. Ultrasound,
I have a question. My period comes every month like clock work o.k I had my last period on September 1st on October 6th I took a pregnancy test came back positive based on the first day of my last period my due date was June 8th.

At my first doctors appointment I was given a ultrasound and that put my due date at June 14th which is impossible cause that would put the first day of my last period at September 7th. My period was leaving so how is that and how off can the ultrasound be?

ANSWER

Hello
An early ultrasound dating a pregnancy is quite accurate. It may be a few days off one way or another, as it is not an exact science. Your first ultrasound was not "off" and it does not not change the first day of your last period–– what it does mean is that you may have ovulated late in your cycle which would mean you are not as far along as you thought.
This would explain the weeks difference with your ultrasound.
If you have any more questions just let me know.

With Regards,
-- Jane Foley RDMS

Jane Foley

Jane Foley has worked as a Sonographer (Ultrasound Technologist) since 1979. Jane has lived and worked in many parts of the world including Saudi Arabia. She is a Registered Diagnostic Medical Sonographer or RDMS. She pulls a wealth of information from her experience in the field of Radiology and her interactions with such a broad cross-section of cultures she has visited. She now makes her home on the island of Maui with her English husband, Michael.