Dating ultrasound measurements

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.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.

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.

Gestational age, synonymous with menstrual age, is defined in weeks beginning from the first day of the last menstrual period (LMP) prior to conception.

Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations.

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.

Fetal growth retardation or macrosomia may be missed or incorrectly diagnosed owing to errors in gestational age assignment.

Interpretation of antenatal biophysical testing (non-stress tests and biophysical profiles) may be subject to variation with gestational age as well.

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.

It is identified by transabdominal ultrasound as early as 5 weeks' gestation and may be seen as early as 4 weeks' gestation by transvaginal ultrasound.

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