Fetal measurements play an important role in second and third trimester ultrasound. Two of the key uses of measurements are assessment of gestational age and estimation of fetal weight.

Sonographic assessment of gestational age becomes progressively less accurate as pregnancy progresses because of increasing biologic variability in fetal size. Thus, fetal measurements should be used to determine gestational age only at the initial sonogram during the pregnancy. At subsequent scans, the best estimate of gestational age is the age determined at the initial scan plus the number of intervening weeks.

Fetal weight estimation is generally reserved for scans done during the third trimester. Once the weight has been estimated, it is then compared to norms for gestational age in order to determine whether the fetus is appropriate in size (generally taken to be a weight between the 10th and 90th percentiles), small, or large for gestational age. Small fetal size may indicate growth restriction due to placental, chromosomal, or infectious causes and should prompt further assessment. Large fetal size may affect decisions regarding route of delivery.

The fetal body parts whose measurements are generally used for these purposes in the second and third trimesters are the head, abdomen, and femur. The method for measuring these structures is presented below. While measurements of other fetal parts are used for several diagnostic purposes, including diagnosis of urinary tract dilation, cerebral ventriculomegaly, skeletal dysplasias, and aneuploidy, these applications of fetal biometry are covered in other sections of this book and are not discussed here.

Measurements of the fetal head (Figure 18.1.1) are obtained on an image in the axial plane that includes the thalami and cavum septum pellucidum, with the falx in the midline. The head should lie horizontally on the image, with its long axis at or close to 90 degrees to the ultrasound beam. The measurements that can be obtained from this image include:

Measurements of the fetal abdomen (Figure 18.1.2) are obtained on a transverse plane through the upper abdomen that includes the stomach and a portion of the intrahepatic umbilical vein, ideally at its junction with the left portal vein. The abdomen should be round, with its width equal or close to the anterior-to-posterior diameter. The measurements that can be obtained on this image include:

Measurement of the fetal femur (Figure 18.1.3) is performed on an image in which the bone lies horizontally, with the ends of the ossified portion (representing the diaphysis) clearly seen. The femur length is measured with calipers placed at the ends of the diaphysis. It is important not to include the cartilaginous epiphysis at the proximal femur or the cartilaginous condyles at the distal femur, whose surfaces may be seen as a linear projection from the diaphysis.