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.