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Is this true if you dont know when your LMP was?

When you have a dating ultrasound you are given an EDD (estimated date of delivery)

If you count back 40 weeks from this date it should be about when your last period was. If you do not have a regular cycle they may not correlate exactly.

38weeks back from your EDD is approximately when you conceived.

Answer Question

Asked by Anonymous at 10:25 AM on May. 1, 2009 in Pregnancy

Answers (7)
  • 40 weeks is counting when your period started. 38 weeks is when you got pregnant. It's odd and makes no sense. This is how women get confused about how far along they are. They count the 2 weeks you weren't pregnant as a means of that preparing your body for pregnancy. It's goofy I know. So if you are wondering about your cycle for due date it's 40 weeks. If you are wondering when conception was is 38 weeks back from due date.

    Answer by lilbit022009 at 10:29 AM on May. 1, 2009

  • Yup lilbit got it right! 38 weeks is when you concived, and 40 is from the first day of your missed period. It is crazy but it all works out. That is why they say you can deliever with no problems at 37 weeks because they can be off by a week or two if you dont know when you last missed period is, and even if you do, but that 37 weeks gives them a good heads up because they could be wrong.

    Answer by babymaddy at 10:44 AM on May. 1, 2009

  • Correct, and if your cycle is very irregular that part can be way off. I had very irregular cycles, and according to my last menstrual cycle my first daughter would have been due in June, but after the dating U/S they gave me a due date the 3rd week of July which is when she was actually born.

    Answer by TweenAndTwinMom at 10:55 AM on May. 1, 2009


    Answer by codysmama2609 at 12:36 PM on May. 1, 2009

  • ^^^ that is a cool site... give you a day by day thing and even show you pics of how your baby is progressing. when you enter tri's and all that

    Answer by codysmama2609 at 12:37 PM on May. 1, 2009

  • I came across that site yesterday ( and i thought it was pretty neat. very helpful.

    Answer by kaf1987 at 12:44 PM on May. 1, 2009

  • If you have irrgeular cycles then I would count back 38 weeks from EDD and take that as date of conecption. Keep in mind that the EDD can be off by a few days or week depedning on haw far along you are when the US is done. An average pregnancy is 36-40 weeks from conception which is typicall 38-42 weeks from LMP.

    Answer by aeneva at 1:16 PM on May. 1, 2009

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