Detection of the small-for-dates twin fetus by ultrasound.

As methods for identifying the small-for-dates fetus in twin pregnancies, serial ultrasonic cephalometry and a new two-stage ultrasound examination schedule have been evaluated and compared. Serial cephalometry is associated with technical and interpretative problems, and only 24 (56 per cent) of 43 small-for-dates fetuses in 66 retrospectively studied twin pregnancies showed detectably abnormal biparietal diameter growth. The two-stage schedule comprises an accurate assessment of gestational age in early pregnancy, and determination of the product of the crown-rump length and trunk area at 34 to 36 weeks; this identified all 15 small-for-dates fetuses in 21 prospectively studied twin pregnancies, and the method offers several other advantages over serial cephalometry.