Ultrasound reference values of fetal basic biometry during second and third trimester: presentation of a population nomogram

Objective: To present a nomogram of basic fetal biometry, namely, biparietal diameter, head circumference, abdominal circumference, femoral length, as well as estimated fetal weight, in the second and third trimesters of pregnancy. Methods: A descriptive, cross-sectional study that included 1564 ultrasound studies of prenatal diagnosis, with gestational age from week 18 to week 36+6d, carried out over a period of 10 years. The measurement of each of the biometric parameters was determined to establish the corresponding nomogram. Results: 470 pregnant women (30.10%) were in the age group between 18-22 years old and 51.74% (808 pregnant women) were primigravid. The biparietal diameter showed an average growth of 2.8 mm per week (r = 0.9478), the head circumference presented an average growth of 9.3 mm (r = 0.9573), the abdominal circumference increased an average of 9.94 mm per week (r = 0.9436) and the length of the femur increased with an average of 2.38 mm per week (r = 0.9513). Fetal growth derived from estimated fetal weight ranges from 249.4 grams at week 18 to 2770 grams at week 36 (Pearson’s correlation coefficient 0.9579). Conclusions: In normally developing fetuses of low-risk pregnant women, the basic biometric parameters, as well as the estimated fetal weight, show constant growth and a positive linear correlation in relation to gestational age. The measurements obtained in this study provide a statistically reliable population nomogram.