2.1 Study Population
Based on fetal ultrasonographic biometry electronic datasets, a retrospective longitudinal study was conducted among pregnant women from the prenatal diagnosis department of International Peace Maternity & Child Health Hospital, Shanghai, China. Those pregnant women who delivered twins as well as singleton between January 2016 and December 2019 were included.
This study would attempt to construct ultrasonic biometry standards to monitor fetal growth for twin pregnancy, only delivery at or beyond 34 weeks of gestation, along with at least two sets of measurements during the whole pregnancy, were considered to be qualified. Exclusion criteria were listed as the followed: uncertain chorionicity; monoamnionicity; key information deficit, mainly gestational age or ultrasonic measurements being unavailable; spontaneous or iatrogenic reduction from a multifetal gestation; fetal death; fetal structural or chromosomal anomalies; occurrence of twin-to-twin transfusion syndrome or twin anemia-polycytemia sequence; selective fetal growth restriction or a birthweight below the 3th percentile for the national birthweight charts25; preexisting maternal disease such as hypertension, diabetes, or renal and autoimmune disorders; and the development of obstetric complications such as gestational hypertensive disease and diabetes. In addition, only very minority (n=23) was MCDA twins conceived by ART, therefore, they were not included in the final analyses; and this study focused on spontaneously conceived monochorionic diamniotic (SC-MCDA), spontaneously conceived dichorionic diamniotic (SC-DCDA) and assisted reproductive technology dichorionic diamniotic (ART-DCDA). While for singleton pregnancy, only delivery at or beyond 37 weeks of gestation were selected, and the inclusion/exclusion criteria were defined in accordance with those of twin pregnancy where appropriate.
Gestational age was calculated by the date of ovulation, the date of embryo transfer, last menstrual period, or the crown–rump length, as appropriate. The diagnosis of chorionicity was based on the number of gestational sacs at 7-8 weeks of gestation and “T sign”, or “lambda sign” obtained by ultrasonography at the first trimester.
Maternal age at delivery was grouped into two categories: <35 vs. ≥35years, and ≥35years was defined as advanced maternal age. Maternal prepregnancy body mass index was grouped into four categories: <18.5, 18.5-23.9, 24.0-27.9, and ≥28.8.