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Methods for determining gestational age


Several methods are available for determining gestational age. These tools can be used either alone or in combination.


  • Last menstrual period: Many women are able to reliably date their pregnancies by identifying and reporting the first day of their last menstrual period. This type of gestational age assessment is considered to have an accuracy of +/- 2 weeks

  • Bimanual exam: Bimanual examination can be used to estimate first trimester uterine size and thereby assess gestational age. The accuracy of this estimate is generally considered to be +/- 2 weeks because uterine size is affected by fibroids, uterine malposition (i.e. retroverted uterus), multiple gestation, and maternal obesity.

  • Serum ß-hCG Testing: Mean serum hCG levels have been shown to be highly correlated with gestational age during early pregnancy. If pregnancy has occurred, ß-hCG can be detected as early as eight days after the LH surge. The ß-hCG concentration in a normal intrauterine pregnancy rises in a curvilinear fashion during the first six weeks of pregnancy at which time it plateaus at approximately 100,000 IU/L. The mean doubling time for the hormone is from 1.4 to 2.1 days. The ß-hCG concentration rises at a much slower rate in most, but not all, ectopic and nonviable intrauterine pregnancies.

  • Ultrasound: Ultrasound is the most accurate method of dating a woman's pregnancy. Ultrasound measurements of crown-rump length, fetal femur length, head circumference, abdominal circumference, and biparietal diameter may be used to assess gestational age. The combination of several measurements is termed fetal biometry. Sonographic estimation may be particularly important when menses are irregular, the LMP is unknown, or an ectopic pregnancy is suspected. Ultrasound also may establish a pregnancy's duration when the uterine size estimated on bimanual examination differs from that predicted by menstrual dating.


If ultrasound equipment is not available or is prohibitively expensive, other methods of dating pregnancies are acceptable when determining eligibility for medication abortion.


If you have questions about medication abortion, please visit our frequently asked questions section.


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