Get your health question answered instantly from our pool of 18000+ doctors from over 80 specialties

170 Doctors Online
Doctor Image
Dr. Andrew Rynne

Family Physician

Exp 50 years

I will be looking into your question and guiding you through the process. Please write your question below.

How is the due date calculated?

Answered by
Dr. John Borowski

General & Family Physician

Practicing since :2001

Answered : 41 Questions

Posted on Mon, 1 Sep 2014 in Women's Health
Question: lnmp and early ultrasound match correctly. then my 18 week ultrasound was off 2 days ? why how can I know when I ovulated?
Answered by Dr. John Borowski 1 hour later
Brief Answer:
Due date goes according to early ultrasound/LNMP

Detailed Answer:
Hi there, I’m Dr John Borowski. I’m a board certified family physician and I’d be glad to try and help you by answering your question.

Your expected due date is 40 weeks from the first day of your last normal menstrual period. If this date was consistent with the dating of your early ultrasound then this is the correct due date for your pregnancy.

The 18 week ultrasound is performed to look at the development of the fetus and is not an accurate way of dating a pregnancy. It can be off by up to two weeks in either direction. Therefore, the “date” given to you at this ultrasound is only an approximation, and does not replace the date according to your LNMP and early ultrasound.

As for when you actually ovulated – i.e. the approximate day of conception - that would have been around two weeks after the first day of your LNMP, assuming a normal 28 day cycle.

I hope I have been able to help you with your query. If you have any further questions I’d be more than happy to help.
Above answer was peer-reviewed by : Dr. Vinay Bhardwaj

The User accepted the expert's answer

Share on