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How accurate are ultrasounds? I am 29 weeks pregnant, if I count back 29 weeks, is that when I had conceived?

Ultrasound, conception date.

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Asked by Cbann02 at 11:26 PM on Mar. 25, 2009 in Pregnancy

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Answers (5)
  • Ultrasounds are just about as accurate as it gets. If your ultrasound says you are 29 weeks it you probably got pregnant 29-30 weeks ago (it takes up to 3 days for the egg and sperm to join and form a fetus)

    Answer by imamommmmyyy at 11:30 PM on Mar. 25, 2009

  • mine was off by 1 day..pretty darn close

    Answer by ronjwake at 11:37 PM on Mar. 25, 2009

  • After the first trimester, ultrasounds are notoriously inaccurate and can be off by two weeks or more in either direction. This is because after the first trimester genetics vary the rate of development and size of the fetus (this is also the reason why using ultrasounds to determine size is a bad idea -- they are notoriously inaccurate). Ball park for an EDD is the best you can get if you don't know conception date for sure.

    Nevery rely on an ultrasound for dates or size estimation. This is well known problem with them.

    Answer by amileegirl at 11:38 PM on Mar. 25, 2009

  • Pretty close...could be off a couple weeks tho...cuz they go by measurements and babies vary in size even before there born.

    Answer by Jacemommy31506 at 12:32 AM on Mar. 26, 2009

  • Ultrasounds in later pregnancy go by size, not development. So no, they are not accurate and are usually off by 1-2 weeks. THe first ultrasound if done about 8-10 weeks is accurate. However, you would count back 27 weeks because the doctor adds 2 weeks. You are "pregnant" before conceptoin.

    Answer by Anonymous at 1:05 AM on Mar. 26, 2009

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