How to Calculate Weeks in Pregnancy

The news of a pregnancy is very exciting. When you finally let people know you are expecting, the first thing nearly everyone will ask is, "When is the baby due?" While it is true that the average human gestation period is nine months, that won't apply to everyone's pregnancy, and maybe not yours. During your pregnancy, your body changes considerably, and there is much to learn and do before the baby arrives. You and your obstetrician or midwife will want your estimates to be as close to your actual delivery date as possible.

Things You'll Need

  • Yearly calendar (two, if your pregnancy begins after February)
  • Pencil/pen

Instructions

    • 1

      Determine the date your last menstrual period started, and note it on your calendar as "Start." This date determines the estimated due date.

    • 2

      Count forward 40 weeks from the start date, and mark the calendar "Estimated Due Date." In reality, most women will have their babies up to two weeks later than the estimated delivery date because of delays in the egg's fertilization.

    • 3

      Make two additional marks on the calendar, one two weeks earlier than the "Estimated Due Date," and one two weeks later than the "Estimated Due Date." You are now looking at a delivery date range of 38 to 42 weeks, which is what is considered perfectly normal for the vast majority of deliveries.

    • 4

      Talk with your health care provider during your prenatal visits to fine-tune your delivery due date estimate. Developmental milestones in both you and your baby will provide significant clues as to how far along you truly are in your pregnancy, and you can adjust the dates on your calendar as you go along.

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