In the past gestational age was established by a combination of the historical information and the physical examination. 4, 5 Ultrasound is a reliable method for establishing the length of pregnancy and in this way can improve obstetric care. Likewise, timing of repeat cesarean section requires accurate assessment of dates. Proper decisions regarding presumed preterm labor or postdate pregnancies are only possible when gestational age is accurately estimated. Obstetric management is also dependent on gestational age. Fetal heart rate reactivity and fetal breathing develop with advancing gestational age therefore, the absence of these biophysical parameters may be interpreted as abnormal for fetuses in whom the gestational age has been overestimated. Interpretation of antenatal biophysical testing (non-stress tests and biophysical profiles) may be subject to variation with gestational age as well. Fetal growth retardation or macrosomia may be missed or incorrectly diagnosed owing to errors in gestational age assignment. Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of gestational age.
Again, inaccurate assessment of gestational age will lead to errors in assessing the severity of fetal sensitization by the delta OD 450. 2, 3 Test results are interpreted based on Liley's zones relative to gestational age. 1 Similarly, the magnitude of increased optical density above baseline at 450 nm (delta OD 450) by amniotic fluid spectrophotometric measurement is used to predict the severity of fetal hemolytic disease in pregnancies complicated by rhesus isoimmunization. Specifically, the level of α-fetoprotein in both amniotic fluid and maternal serum is related to gestational age and when dates are inaccurate test results will be incorrect and misleading. For example, antenatal test interpretation may be dependent on gestational age. Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations. Gestational age, synonymous with menstrual age, is defined in weeks beginning from the first day of the last menstrual period (LMP) prior to conception.
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