Customised fetal growth curves in twin pregnancies: an Italian multicentre study

Objectives To construct reference limits for gestation of fetal biometric parameters stratified by chorionicity and customised for obstetrical and parents' characteristics. Methods Fetal biometric measurements derived from serial ultrasound examinations obtained from uncomplicated twin pregnancies delivering after 36 weeks of gestation were collected by 19 Italian fetal medicine units under the auspices of the Italian Society of Ultrasound in Obstetrics and Gynecology (SIEOG). The measurements acquired in each fetus at each examination included biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Multilevel linear regression models were used to adjust for the serial ultrasonographic measurements obtained and the clustering of each fetus in twin pregnancy. The impact of maternal and paternal characteristics (height, weight, ethnicity), parity, fetal sex and mode of conception were also considered. Models for each parameter were constructed stratified by fetal chorionicity and compared to our previously constructed growth curves from singleton. Results The dataset included 1781 twin pregnancies (dichorionic 1289; monochorionic diamniotic 492) with 8923 ultrasonographic examination with a median of 5 (range 2–11) observations per pregnancy. Growth curves of twin pregnancies differ from those of singletons, and differences were more marked in MCDA and during the third trimester. A significant influence of parents' characteristics was found as in singleton pregnancies. Conclusions Curves of fetal biometric measurements in twins are influenced by parents' characteristics. There is a reduction in growth rate during the third trimester. The reference limits for gestation constructed in this study from uncomplicated twin pregnancies may provide a useful tool for the identification of fetal growth abnormalities in twin pregnancies.