Obstetric management is also dependent on gestational age.Proper decisions regarding presumed preterm labor or postdate pregnancies are only possible when gestational age is accurately estimated.Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations.For example, antenatal test interpretation may be dependent on gestational age.Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of gestational age.Fetal growth retardation or macrosomia may be missed or incorrectly diagnosed owing to errors in gestational age assignment.
Reliance was placed on the menstrual history and the maternal sensation of fetal movement (“quickening”).
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.
The gestational sac is the first identifiable structure routinely imaged in the first trimester.
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.
Hertz and co-workers reported that menstrual history was considered reliable in only 18% of women.