Factors affecting the accuracy of ultrasonographical fetal weight estimation in twin pregnancies

To determine the factors affecting the accuracy of ultrasonographic weight estimation in twins.

152 sets of twins delivered vaginally, were included. Effects of fetal weights, inter-twin weight discordance, chorionicity, early rupture of membranes, intrauterine growth restriction, and presentations of twins on errors of estimated fetal weights were evaluated. The primary measures of estimated fetal weight accuracy compared were mean-percentage-error and the standart deviation (SD) of percentage errors.

Mean percentage errors for the first fetus (8.13 ± 6.82) and the second fetus (8.07 ± 6.88) were similar (p = 0.64). Random errors of both fetuses were also similar (p = 0.78). If one of the fetuses had IUGR, the percentage error and also the random error of that fetus would increase significantly. Different presentations and fetal gender combinations were similar for both types of errors of fetal weight estimation. A weak negative lineer relationship was found between the weight of the first fetus and its percentage error (r = -0.27, p = 0.04). A similar relation was present between the weight and percentage error of the second fetus (r = -0.29, p = 0.03). Percentage errors and also random errors of both fetuses were significantly higher if severe discordance was present between twins (p = 0.01 and p = 0.02, respectively).

IUGR, fetal weights, and inter-twin discordence are the factors affecting the accuracy of weight estimation by ultrasonography.