Accuracy of ultrasound dating
In vitro fertilization, with known date of conception, is likely the most accurate means of predicting gestational age (±1 day).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.Both parameters are useful because each measures a different aspect of the first-trimester pregnancy and may be used at different times during the first trimester.The gestational sac is the first identifiable structure routinely imaged in the first trimester.
Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of 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.
Obstetric management is also dependent on gestational age.
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.
Test results are interpreted based on Liley's zones relative to gestational age.