Comparison of second trimester biometry in singleton and twin pregnancies conceived with assisted reproductive techniques.

The objective of this study was to investigate the size of singleton vs twin pregnancies at the time of a second trimester dating scan. The analysis included 86 infants from 63 pregnancies achieved with assisted reproductive techniques, comprising 40 singletons and 46 twins. Measurements of second trimester biparietal diameter (n = 85) and femur length (n = 74) were plotted against the precisely known gestational age. A common regression line was calculated for each parameter and the residuals for singletons and twins were compared. Gestational age and weight at birth were also analysed for each group. There was no significant difference between singletons and twins in biparietal diameter or femur length in second trimester. In contrast, twins had a lower mean birthweight, gestational age at birth, and weight-for-gestational age centile compared with singletons. Singleton babies from these pregnancies had an average birthweight centile of 49.8% (i.e. close to the median for spontaneously conceived pregnancies in our population). We concluded that the same pregnancy dating charts can be used for singletons and twins. At corresponding gestational age, twins are smaller than singletons at birth because of slower growth in the third trimester.