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How is conception date figured out when you have an irregular period?

I'm pregnant. The doctors are saying I'm exactly 16 weeks and two days. My last period was June 7th, 2009, but the date I supposedly got pregnant is July 20th, with that making the conception date August third. Does this sound right

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Asked by Anonymous at 12:49 AM on Nov. 11, 2009 in Pregnancy

Answers (6)
  • IDK, but when you go in for your ultrasound at 20 weeks they will have a due date according to the measurements of the baby.

    Answer by Anonymous at 12:53 AM on Nov. 11, 2009

  • The doctors know what their talking about! trust them! I have a very irregular period too and they were quick to tell me my due date then I got a sono. and they measured him. they were a week off but thats normal because they count from the first day of your last period. No worried dear! they are most likely right! Congrats!

    Answer by Alysia_Moore at 12:55 AM on Nov. 11, 2009

  • 20 week estimates of due date can be off by a week or more - after the 12th week of pregnancy, babies grow in less predictable ways and it becomes harder and harder to estimate EDD on the basis of an U/S alone.
    Doctors can guess-timate an EDD based on your LMP, along with a pelvic exam, the position and size of your uterus, and any other fertility factors that may be in play. For example, I didn't know when I got pregnant, I hadn't had a period since before my son was born. But based on when I started weaning him as well as the size and position of my uterus, my CNM was able to make a fairly reliable estimate of when I'd gotten pregnant. The U/S I had at around 8 weeks pregnant confirmed the due date range she'd arrived at with some old fashioned detective work.

    Answer by eema.gray at 1:01 AM on Nov. 11, 2009

  • However, keep in mind that the EDD calculations made in most doctor's offices assume that you have a 28 day cycle and that you ovulate on the 14th day of that cycle, every cycle. This is not necessarily true.
    For example, my cycle is about 40 days long and I ovulate later into it than average. The standard EDD calculation will not "work" for me because it doesn't take into account the duration of my cycle. Therefore, my babies are always over a week "late" because I have a longer than average cycle. If the particulars of my cycle are taken into account, my babies only arrive just a few days over 40 weeks.

    Answer by eema.gray at 1:04 AM on Nov. 11, 2009

  • Are they basing that gestational age on an ultrasound? If so, they are probably pretty accurate - at least within a week. If you have very irregular cycles, it's definitely possible to have gotten pregnant that far after your last cycle. They will check again at your 20 week ultrasound to confirm the date. Good luck, and congrats! :)

    Answer by Brieshon at 2:48 AM on Nov. 11, 2009

  • they just guess. either by ultrasound or palpations. just make sure the don't try to induce you in case they are off. just wait till baby comes on his or her own naturally. congrats!

    Answer by Anonymous at 9:08 AM on Nov. 11, 2009

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