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are all pregnancies and conception dates estimates?

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I'm just wondering what the chances of my due date being incorrect are...My pregnancy edd by the doc and mw before my scan was 7/8 my 1st u/s then dated me at the 5/8 which brought me forward 2 days...However on the calculators on the internet it has gave me that due date based on a 28 day cycle...Just to make things easier i have never had a 28 day cycle and ovulated on day 14..if i change the date on the calculator to the only time i could possibly of fell pregnant that month it tells me i would be due on the 15/8 when i spoke to my mw about this she said they will just go off the scan reports...are all pregnancies and conception dates estimates? If so could wouldnt this make the weeks of pregnancy i was less? i am now 26weeks and i'm sure i should only be 24/25 with a due date later than the 5/8

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replied May 5th, 2011
Pregnancy Q and A Answer A22652
Welcome to ehealthforum,
Ultrasound scan dates are more reliable. Your ovulation day can vary based on hormonal and stress levels. Sperms can stay alive for 3-4 days inside of you and egg once released from ovary can stay alive for 6-7 days. The exact time of fertilization cannot be predicted. The ultrasound scan gives you the expected due date based on the growth & developmental stage of the baby (which goes on according to fixed schedule each week of pregnancy). Early scans can give variation of +/- 5-7 days and late scans are more accurate. Go ahead with a 'dating scan' done for your satisfaction, it will show the probable conception date.
Take care.

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