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.

In vitro fertilization, with known date of conception, is likely the most accurate means of predicting gestational age (±1 day).

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.

The gestational sac is an echo-free space containing the fluid, embryo, and extraembryonic structures.

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.

Proper decisions regarding presumed preterm labor or postdate pregnancies are only possible when gestational age is accurately estimated.

Likewise, timing of repeat cesarean section requires accurate assessment of dates.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.However, both the history and the findings on physical examination are fraught with error, even in the best of circumstances (Table 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.Test results are interpreted based on Liley's zones relative to gestational age.The sac is measured inside the hyperechoic rim, including only the echo-free space (Fig. The gestational sac is imaged first in the longitudinal plane, obtaining long axis and anteroposterior measurements perpendicular to each other. The longitudinal diameter is measured between the small crosses (1) and the anteroposterior diameter is measured between the larger crosses (2). Transverse image obtained at the level of the anteroposterior diameter.