The layers of the human amnion and chorion were examined by light and transmission electron microscopy. Comparisons among different anatomical sites with respect to full-term and prematurely ruptured membranes indicate that (a) the thickness of the membranes is reduced near the rupture point; (b) intercellular canals near the implantation site become dilated and branched; (c) the trophoblast layer of full-term membranes is thinner and with more degenerating cells; and (d) the fibroblast and spongy layers have fewer collagenous fibers and less organization near the rupture site. These findings suggest that, although cellular activity is maintained in prematurely ruptured membranes, the mainly collagenous extracellular matrix undergoes marked disorientation. If this occurs too early in gestation, it may lead to premature rupture.