How to Calculate Human Gestation

Calculating gestation is not an exact science, and there is a large margin of error. According to the American Pregnancy Association (APA), roughly 95 percent of pregnancies do not end on their estimated due dates. According to Dr. Joseph Woo's article on the Obstetric Ultrasound website, ultrasound can be used to confirm and date pregnancy as early as four and one-half to five weeks gestation.

Instructions

  1. Calculating Human Gestation Using LMP

    • 1

      Ascertain your LMP-the first day of your last menstrual cycle or period.

    • 2

      Count 11 to 21 days from the first day of your last LMP to approximate a conception date.

    • 3

      Count 40 weeks from the first day of your last LMP to approximate a tentative due date.

    • 4

      Taking measurements, such as the crown-to-rump measurement, using ultrasound early in a pregnancy can help determine gestational age. However, Woo cautions that measurements taken toward the end of a pregnancy may not be as accurate and could indicate growth restriction.

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