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.