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How is the due date calculated?

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General & Family Physician
Practicing since : 2001
Answered : 41 Questions
lnmp and early ultrasound match correctly. then my 18 week ultrasound was off 2 days ? why how can I know when I ovulated?
Posted Sat, 30 Aug 2014 in Women's Health
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.
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