Premature rupture of the membranes (PROM) is still an unsolved problem in obstetrics. Neither prevention nor diagnosis nor therapy is readily available. The amniotic sac is stabilized mechanically by a dense network of collagenous fibres; in the event of PROM, the collagenous mesh is dissolved and the thickness of the amniotic sac is reduced. In a prospective study involving 32 patients, we measured the thickness of the membranes in vitro by high-frequency ultrasound and by light microscopy of histological sections, between gestational weeks 28 and 32. We compared 18 patients with premature rupture of the amnio-chorionic membrane with 14 patients following induction of labour, with intact membranes at the same gestational age in vitro. We were able to show that the membrane in the PROM group (0.54 +/- 0.9 mm) was markedly thinner with fewer collagenous fibres than in the control group (0.74 +/- 1.01 mm). The application of high-frequency ultrasound in vivo in PROM may become helpful for the prediction of PROM; further studies are therefore desirable.