The standard deviations of biometric measurements taken from normal pregnancies increase as pregnancy progresses. Thus, fetal biometric images are most useful for obtaining an accurate estimate of fetal age when performed in the first trimester but at least by 20 weeks of pregnancy, with a reasonably accurate estimation as late as 28 weeks. The American College of Obstetricians and Gynaecologists considers pregnancies "suboptimally dated" in the absence of an ultrasound examination before 22 0/7 weeks of gestation confirming or revising the EDD.
If the first ultrasound examination is performed after 22 weeks of gestation and discordant with menstrual dates, it is inappropriate to rely solely on a single set of the usual biometric parameters for assessment of gestational age. In these cases, serial measurements three to four weeks apart can be helpful. Normal interval growth supports the…
