The advent of ultrasound has allowed a more direct means of assessing fetal structures and development.Measurements of a wide variety of parameters have been devised to establish gestational age.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.Hertz and co-workers reported that menstrual history was considered reliable in only 18% of women.In another report, even among women with known LMP, neonatal age assessment differed markedly from that assigned by certain menstrual dates in 15%.Bimanual examination in the first trimester may be accurate within ±2 weeks; however, fundal height measurement, which is more commonly used to assess gestational age, is only accurate within ±4 to 6 weeks.For example, antenatal test interpretation may be dependent on gestational age.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.
Those parameters most commonly measured include biparietal diameter, In the first trimester, the gestational sac mean diameter and crown-rump length are used to establish fetal age.
Clearly, the inaccuracies of history and physical examination may limit their usefulness in assessment of gestational age.
Methods that assess the time of ovulation or conception can accurately establish gestational age.
Again, inaccurate assessment of gestational age will lead to errors in assessing the severity of fetal sensitization by the delta OD 450.
Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of gestational age.