[Fetal causes of the onset of labour. A kybernetic model (author's transl)].
The question of initiation of labour is not yet solved. A fetal fixing of the date would be ingenious. On the one hand the fet is endangered by prematurity, on the other hand by placental insufficiency in postmaturity. Fetuses with comparatively lighter placenta remain in average shorter in utero until spontaneous initiation of labour than those with comparatively heavier placenta. This fact leads to the assumption of a relative placental insufficiency as a determining factor of the onset of labour. A further inquiry demonstrates, that babies born after premature termination of pregnancy about the term - either by induction of labour or by primary sectio caesarea - are heavier and longer than those born after spontaneous labour. This leads to the assumption of a prenatal weight loss of the infant before spontaneous initiation of labour. This weight loss is caused by diminution of water content and relative placental insufficiency. This relative placental insufficiency also leads to a diminution of amniotic fluid, which is swallowed by the "hungry" fet in a greater amount. Altogether a diminution of uterine volume is resulting, which is accomplished by a diminution of uterine wall tension. The coordination of uterine activity, which precedes delivery, is combined with the ripening of the cervix. The ripening of the cervix also leads to a retraction of myometrium and thus to a further diminution of uterine wall tension. It is concluded, that a diminution of uterine volume, causes by fetal weight loss and diminution of amniotic fluid, leads to a reduction of uterine wall tension, which is supported by the ripening of the cervix. This reduction of uterine wall tension is the precondition of the increasing coordination of activity, which precedes delivery.