The advent of ultrasound has allowed a more direct means of assessing fetal structures and development. TABLE 1.Clinical Predictors of Gestational Age However, in most pregnancies, the date of ovulation or conception cannot be as accurately predicted as outlined above and gestational age must be established by other methods. In vitro fertilization, with known date of conception, is likely the most accurate means of predicting gestational age (☑ day). Ovulation induction with agents such as clomiphene citrate and Pergonal, also accurately predicts gestational age.
11, 12, 13, 14 Timed ovulation, either by basal body temperature recording or semiquantitative assessment of luteinizing hormone surge, predicts gestational age within ±4 to 6 days. Methods that assess the time of ovulation or conception can accurately establish gestational age. Clearly, the inaccuracies of history and physical examination may limit their usefulness in assessment of gestational age. 10 Bimanual examination in the first trimester may be accurate within ☒ weeks however, fundal height measurement, which is more commonly used to assess gestational age, is only accurate within ±4 to 6 weeks. 8 Physical examination also tends to be inaccurate, especially with advancing gestational age. In another report, even among women with known LMP, neonatal age assessment differed markedly from that assigned by certain menstrual dates in 15%. Hertz and co-workers 9 reported that menstrual history was considered reliable in only 18% of women. 6, 7 Some of the reasons for this uncertainty include oligomenorrhea, metrorrhagia, bleeding in the first trimester of pregnancy, pregnancy following use of oral contraceptives or intrauterine devices, and becoming pregnant in the postpartum period. 6, 7, 8 It has been estimated that 20% to 40% of women cannot relate the LMP with certainty. However, both the history and the findings on physical examination are fraught with error, even in the best of circumstances ( Table 1). Other factors include assessment of uterine size by bimanual examination in the first trimester, initial detection of fetal heart tones by Doppler (10–12 weeks) or auscultation (19–21 weeks), and uterine fundal height measurement. Reliance was placed on the menstrual history and the maternal sensation of fetal movement (“quickening”). 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.