Starting at 14°/ weeks, equipment software formulas calculate estimated gestational age and fetal weight from measurements of the biparietal diameter, head and abdominal circumference, and femur length (Fig. 10-2). The estimates are most accurate when multiple parameters are used but may over-or underestimate fetal weight by up to 20 percent (American College of Obstetricians and Gynecologists, 2016). Various nomograms for other fetal structures, including the cerebellar diameter, ear length, ocular distances, thoracic circumference, and lengths of kidney, long bones, and feet, may be used to address speciic questions regarding organ system abnormalities or syndromes (Appendix, p. 1266).
The biparietal diameter (BPD) most accurately reflects gestational age, with a variation of 7 to 10 days in the second trimester. The BPD is measured perpendicular to the midline falx in the transthalamic view, at the level of the thalami and cavum septum pellucidum (CSP) (see Fig. 10-2A). Calipers are placed from the outer edge of the skull in the near ield to the inner edge of the skull in the far field. he head circumference (HC) is also measured in the trans thalamic view. An ellipse is
FIGURE 10-2 Fetal biometry. A. Transthalamic view. A transverse (axial) image of the head is obtained at the level of the cavum septum pellucidum (arrows) and thalami (asterisks). The biparietal diameter is measured perpendicular to the sagittal midline, from the outer edge of the skull in the near field to the inner edge of the skull in the far field. By convention, the near field is that which is closer to the sonographic transducer. The head circumference is measured circumferentially around the outer border of the skull. B. Femur length. The femur is measured perpendiCUlar to the femoral shaft, from each diaphyseal end, excluding the epiphysis. C. Abdominal circumference. This is a transverse measurement at the level of the stomach (5). The J-shaped structure (arrowheads) indicates the confluence of the umbilical vein and the right portal vein. Ideally, only one rib is visible on each side of the abdomen, indicating that the image was not taken at an oblique angle.
placed around the outer edge of the skull or the circumference is calculated using BPD and occipital-frontal diameter (OFD) values. he cephalic index, which is the BPD divided by the OFD, is normally 70 to 86 percent.
If the head shape is lattened-dolichocephay, or roundedbrachycephay, the HC is more reliable than the BPD. hese head shape variants may be normal or can be secondary to fetal position or oligohydramnios. But, dolichocephaly can occur with neural-tube defects, and brachycephaly may be seen in fetuses with Down syndrome. lso, with abnormal skull shape, craniosynostosis and other craniofacial abnormalities are a consideration.
he femur length (FL) correlates well with both BPD and gestational age. It is measured with the beam perpendicular to the long axis of the shat. Calipers are placed at each end of the calcified diaphysis and exclude the epiphysis. For gestational age estimation, it has a variation of 7 to 11 days in the second trimester (see Fig. 1O-2B). A femur measurement that is <2.5th percentile for gestational age or that is shortened to :;90 percent of that expected based on the measured BPD is a minor marker for Down syndrome (Chap. 14, p.287). he normal range for the FL to abdominal circumference (AC) ratio is generally 20 to 24 percent. A dramatically foreshortened FL or a FL-to-AC ratio below 18 percent prompts evaluation for a skeletal dysplasia (p. 210).