For women with a menstrual cycle of average length, that day is usually about two weeks before conception, which explains why pregnancies are said to last 40 weeks.
This method doesn't take into account how long your menstrual cycle actually is or when you think you might have conceived.
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Also, when providers all use the same LMP date, we are using one system that provides a standard convention.
The average time between menstrual periods is about 28 days, and ovulation occurs about 14 days before the next period begins.
If you don't have that information, you can work backwards from your due date, with the help of an ultrasound and your health care provider.