Last menstrual period ultrasound dating
Example: LMP = 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. The calculated date (15 February) is a Monday; adjusting to the closest Friday produces 12 February, which is exactly 280 days past 8 May.
If the difference between the two is less than a week apart, she’ll generally go with the date calculated from your last period.
Did you get your very first ultrasound in your second trimester? The most accurate due date predictions come from ultrasounds performed during the first trimester, not the second, according to a study in the American Journal of Obstetrics and Gynecology.
Every time you visit your health care provider, she’ll check your fundal measurement — the distance from the top of your pubic bone to the top of your uterus.
The result is approximately 280 days (40 weeks) from the start of the last menstrual period.
Another method is by adding 9 months and 7 days to the first day of the last menstrual period.
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In a study by Women’s Health (2005), approximately one half of women remember their last menstrual period date correctly.