The creation of twin centile curves for size

Abstract

Objective: To create twin centile charts for size.

Design: Retrospective study.

Setting: Aberdeen Maternity Hospital.

Methods: Ultrasound measurements of abdominal circumference (AC), biparietal diameter (BPD) and femur length (FL) for 1,011 twin pregnancies were extracted from a databank. The methodology is based on that described by Altman and Chitty [Br. J. Obstet. Gynaecol. 101 (1994) 29.]; only one measurement from each twin was used, polynomial regression models were fitted separately to the mean and standard deviation (SD) of the ultrasound parameter as functions of gestational age. The centiles were obtained assuming that at each gestation, the measurements had a normal distribution.

Results: The growth pattern of AC for twins appear to follow closely that of singletons until 32 weeks. Thereafter, there is a gradual but definite fall off in growth away from singleton standards. The pattern of growth of FL is largely similar to that of singletons. From mid to early third trimester, the BPD of twin babies was larger than that of singletons.