Intrauterine fetal growth in concordant twin gestations

We longitudinally assessed intrauterine ultrasonic growth parameters (biparietal diameter, head circumference, abdominal circumference, and femur length) in 60 pairs of concordant twins. Head circumference to abdominal circumference and femur length to abdominal circumference ratios were calculated. Estimated fetal weight curves were created with the formula of Shepard et al., incorporating biparietal diameter and abdominal circumference, as well as that of Hadlock et al., incorporating femur length and abdominal circumference. Biparietal diameter was obtained in only 79% of fetuses, whereas femur length and abdominal circumference were obtained in 96% and 99% of fetuses, respectively. The intrauterine growth of abdominal circumference appears to be linear between 18 and 40 weeks, fitting the simple equation abdominal circumference = -4.5 + 0.97 gestational age (gestational age in weeks). The mean femur length to abdominal circumference ratio is 22.4 +/- 1.5 and appears to be gestational age independent between 20 and 40 weeks. The head circumference to abdominal circumference ratio decreases as gestational age advances in a linear fashion. Estimated fetal weight curves by the formulas of both Shepard et al. and Hadlock et al. fit second-order polynomial equations. Neither formula appears to be superior in estimating fetal weight in twin gestations, although that of Hadlock et al. can be used more frequently since biparietal diameter cannot always be obtained in both twins.