How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Dr. Zach Your Own Question
Dr. Zach
Dr. Zach, OB/GYN (Doctor)
Category: OB GYN
Satisfied Customers: 4521
Experience:  Accomplished Physician and Educator
Type Your OB GYN Question Here...
Dr. Zach is online now
A new question is answered every 9 seconds

Can an ultrasound be off by 3 weeks My due date was 08/01,

Customer Question

Can an ultrasound be off by 3 weeks? My due date was 08/01, 2010, I had my child 07/31/2010. I am concerned because of paternity issues. I already had a man tested, an he was negative. I am worried that the 10/17, guy is the father can he be?By the way my daughter was healthy, weighin 7.2
Submitted: 6 years ago.
Category: OB GYN
Expert:  Dr. Zach replied 6 years ago.

Hello! My name isXXXXX and I will be happy to assist you today.


An ultrasound can be off by three weeks depending on when it was performed. As a general rule of thumb:


1st trimester ultrasounds can be off by about a week

2nd trimester ultrasounds can be off by about 2 weeks

and 3rd trimester ultrasounds can be off by about 3 weeks


Hope that helps. Take care. Please click accept for my time. Thanks.

Customer: replied 6 years ago.
so my baby being born healthy, and close to the due date has nothing to do with it?
Expert:  Dr. Zach replied 6 years ago.
With the ultrasound due date you gave, the probably date of conception is somewhere around November 7th. So depending on when you had the ultrasound, (1st, 2nd, or third trimester) you will be able to figure out roughly when you had sex to result in that pregnancy.
Customer: replied 6 years ago.

i was sure that it was the guy from nov, everyone kept tellin me oct 17 was to far back for a 08/01 due date

Customer: replied 6 years ago.
im not sure of exzctly when i got the first u/s but if it helps on April 5, I was 23wks
Expert:  Dr. Zach replied 6 years ago.
Well if you knew by the time you were 23 weeks then that means you had either a first or second trimester US in which case the error should only be about 2 weeks. So it seems unlikely the October 17 man is the father based on that information.
Dr. Zach and other OB GYN Specialists are ready to help you
Customer: replied 6 years ago.
you have really been a big help, thank you
Expert:  Dr. Zach replied 6 years ago.
Of course, take care :) And congratulations on your healthy baby!