[Behavior of fetal position in the 2d half of pregnancy in labor with breech and vertex presentations].

In a retrospective analysis, presentation behavior during pregnancy of 501 infants with breech presentation at birth was compared with that of a control population (n = 520) with vertex presentation at birth. Since the fetus usually changes position frequently during the sonographic examination prior to the 20th week of pregnancy, and since its presentation behavior is generally unstable, presentation of the fetus was only determined sonographically from this point onward. In both populations it was found in 40% of the cases that the fetus remained in either breech or vertex presentation up to birth; and in both populations there was one change of position to the definitive presentation at birth in 44% of the cases (before the 33rd week of pregnancy in 95%). Seventeen percent of the infants born from breech presentation and 15% of those born from vertex presentation changed position several times during pregnancy. The stability of presentation behavior during pregnancy of infants with breech presentation at birth was similar to that of infants with vertex presentation. At about the 32nd week of pregnancy, 90% of the fetuses in both breech and vertex presentation had assumed their final presentation prior to birth. Even when breech presentation is found between the 33rd and 36th week of pregnancy there is still a 54% probability of a change to vertex presentation at birth. In contrast, the probability of the fetus turning from vertex to breech presentation at birth in this period of pregnancy is only 0.4%.