[Determination of the gestation period by means of ultrasound. Cephalometry in twins (author's transl)].

In 147 twin pregnancies, the biparietal diameter of the skull was determined by means of an ultrasound B-scan method. 574 individual measurements were evaluated. It was found that a marked flattening of the growth curve sets in from the 30th week of pregnancy, compared with single births. The difference amounted to 6 mm in the 40th week of pregnancy. The wavy shape of the mean value curve in twins up to the 30th week of pregnancy, appears particularly remarkable. The reason for this phenomenon could be seen in high-grade placental insufficiency causing spontaneous abortion or premature delivery. Accordingly, an increased incidence of abortion was recorded in the 21st and 25th week, as well as a peak in premature births in the 29th and 30th week of pregnancy. The heads of the twins were not equal in size. The average difference of the diameters of the skull was 6 mm.