Ultrasound assessment of gestational age is feasible in a majority of pregnancies and may be used to establish gestational age with greater accuracy than physical examination.
In the first trimester, gestational sac mean diameter and crown-rump length measurements have become the primary means of evaluating gestational age.
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).
For example, antenatal test interpretation may be dependent on gestational age.
Then, in the transverse plane at the level of the anteroposterior measurement, the width measurement is obtained. The transverse diameter is measured between the crosses.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.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.