The aim of this study was to elucidate whether the mechanical properties of fetal membranes change during late pregnancy and labour. Membranes delivered by elective caesarean section in week 38 showed different load-strain curves to membranes obtained after spontaneous vaginal delivery at term. A major change in mechanical properties was a decrease in strength of the intact chorioamniotic membrane from 1.39 N to 0.98 N (width of biopsy 4 mm), mainly due to loss of strength of the amniotic component, from 1.27 N to 0.72 N. Moreover, the extensibility of the chorionic component (epsilon Fmax) increased after vaginal delivery from 0.42 to 0.54, resulting in a two-component behaviour of the intact chorioamniotic membrane and further decrease of strength. Based on these observations it is suggested that the amnion is attached to the chorion, act biomechanically in parallel and possess relatively high mechanical strength during pregnancy. After vaginal delivery the two membranes are separated, and pronounced changes are induced in their mechanical properties.