<P> 280 days past the start of the last menstrual period is found by checking the day of the week of the LMP and adjusting the calculated date to land on the same day of the week . Using the example above, 8 May 2009 is a Friday . The calculated date (15 February) is a Monday; adjusting to the closest Friday produces 12 February, which is exactly 280 days past 8 May . The calculation method does not always result in 280 days because not all calendar months are the same length; it does not account for leap years . </P> <P> Mobile apps essentially always give consistent estimations compared to each other and correct for leap year, while pregnancy wheels made of paper can differ from each other by 7 days and generally do not correct for leap year . </P> <Ul> <Li> 276 days for both ultrasound - estimated and LMP - estimated gestational age in a US study of 1867 singleton live births . </Li> <Li> 281 days after LMP with a standard deviation of 13 days, was the result of a population - based study of 427,581 singleton births in Sweden . </Li> <Li> 281 days after LMP for first - time mothers and 280 days for all others were the medians found by a 1995 American study of 1,970 spontaneous births . Standard deviation was 7--9 days . </Li> <Li> 282 days after LMP was recommended for cases where LMP is the only known factor, in a study of 17,450 patients combining LMP and ultrasound measurement techniques . </Li> <Li> A median of 288 days (274 days from the date of ovulation) for first - time mothers and 283 days (269 days from the date of ovulation) for mothers with at least one previous pregnancy was found by a 1990 study of 114 white, private - care patients with uncomplicated pregnancies and spontaneous labor . The authors suggest that excluding pregnancies involving complications (that often lead to pre-term deliveries) accounts for the longer periods . </Li> </Ul> <Li> 276 days for both ultrasound - estimated and LMP - estimated gestational age in a US study of 1867 singleton live births . </Li>

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