How to Calculate a Pregnancy Due Date

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The gestation of a human baby is considered to be 40 weeks. However, the average pregnancy lasts anywhere from 37 to 42 weeks. Yes, a normal pregnancy can last longer than nine months. This is often the first of many pregnancy surprises for first-time moms. Since only 5 percent of babies are born on their due date, according to the American Congress of Obstetricians and Gynecologists (ACOG), use this date as a general time line and not as something set in stone.

Step 1

Determine the first day of your last menstrual period. This would be the first day that you had red blood, not brown spotting.

Step 2

Count 40 weeks, or 280 days, on a calendar from the first day of your last menstrual period. Technically, you were not pregnant the first two or three weeks, but it is part of the calculation. Consider it a bonus that you have already finished the first month of pregnancy before you even knew you were pregnant.

Step 3

Use your estimated conception date to determine your due date if your cycle averages 30 days or longer. Add 266 days, or 38 weeks, to this conception date to estimate your due date. Conception would have occurred about 10 to 14 days before your expected next period. You may have felt some unusual cramping or had spotting at this time.

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