How Doctors Estimate a Woman’s Due Date


How much will the baby weigh? How many hours will she be in labor? What day will the baby be born? All of these questions are the basis for some of the most popular pregnancy pools that family and friends love to bet on. The "due date pool" is simply where people guess the date the baby will be born.

Doctors have a method for determining this date, a method that is usually accurate with fourteen days of delivery. The equation is as follows:

LMP + 7 days - 3 months = Due Date

LMP stands for last menstrual period. Doctors will take the first day of a woman's last menstrual cycle and add 7 days. Then they subtract 3 months. Another popular formula takes the LMP and adds 240 days or 40 weeks.

Irregular cycles can throw these formulas off, however. Women who keep accurate records of their cycles and can pinpoint their ovulation dates can increase the accuracy of their predicted due dates. This involves monitoring basal temperatures and various bodily symptoms that signify different times in a woman's cycle. This is usually performed with several months of at-home monitoring.

Another method doctors will use to determine a due date is the size of the baby. Between 8 and 12 weeks, doctors can measure the length of the fetus via ultrasound. This method is believed to be more accurate than formulas based on the date of the last menstrual period.

Only about 5% of women actually give birth on their due dates. Doctors usually reference a 'due month' rather than a specific day, because there is always the possibility for complications. Ask your doctor for more information, and good luck on those pools.

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