Can an Early Ultrasound Be Off By 4 Weeks?

QUESTION

Dear Ms. Ultrasound,
I have a history of irregular cycles (and diagnosed with PCOS), so using my lmp to date my pregnancy was not an option. My LMP was June 26th, 2008.

I took 3 home pregnancy tests prior to a Dr's appointment.. The first one was 6 weeks after my lmp (August 7th- it came back negative) the second was taken on August 12th (negative) and the third was taken August 24th (it came back positive).

At my first doctors visit on August 28th, we did a transvaginal ultrasound that showed no heart beat. I came back for a second ultrasound a week and a half later and the ultrasound confirmed there was a heart beat and that I was 6w5d along with an EDD of April 29th, 2009. I had two more weekly ultrasounds that confirmed my due date (one at 7w5d and another at 9w). Again, I had another ultrasound at 18 weeks and it confirmed my April 29th due date. I was told everything looked normal and healthy. If I understand correctly, a due date of April 29th, would mean I conceived on August 6th 2008.

The reason I am concerned is because I had a one night stand with someone I was not in a relationship with on July 11th, 2008. We used protection except for about 30 seconds in the very beginning.

From what I understand early ultrasounds are the most accurate way to date a pregnancy short of knowing the exact date of conception. I've been told within 3-4 days. If this is the case, I have nothing to worry about and the baby is my husbands, conceived on August 6th. What I need to know is, is there any way that the ultrasounds could be off by about 4 weeks? I am very concerned about the accuracy of my date!!! Please help me to understand if I can rest easy with the dating of my ultrasound and the conception date I calculated? thank you.

ANSWER

Hi, From your last period date, your due date should be April 2nd with a conception date of July 10th which does not correlate with your ultrasound!!!

From your ultrasound dates with a due date of April 29th your conception date should have been around Aug 6th.

An ultrasound would not be 4 weeks off so you can relax about paternity.
Please note that if paternity is going to be an issue in the back of your mind for years to come, you may want to consider a paternity test once your child is born.

With Regards,
-- Jane RDMS

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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.