The objective of the study was to evaluate the relative contributions of amnion and chorion to the strength of fetal membranes and to correlate these findings with gestational age.Fetal membranes from 78 pregnancies were tested for biaxial puncture force using a blunt, instrumented probe with a low-force load cell connected through a load cell conditioner to an oscilloscope. The average of 2 to 4 tests performed on independent regions of the membrane was recorded. Means and SDs were calculated through the gestational age ranges of less than 32, 32 to 36, or 37 weeks or longer. Linear regression analysis was performed across gestational age after grouping data by labor and mode of delivery.There were trends toward decreasing puncture force with gestational age for both chorioamnion and amnion for both vaginal deliveries and cesarean sections. The trends were significant by linear regression for labored deliveries but not unlabored cesarean sections for both chorioamnion and amnion alone. There was no trend in chorion puncture force with either gestational age or delivery mode and the mean puncture force values were, on average, half those for the amnion.The amnion is significantly stronger than the chorion when subjected to biaxial strength testing. The amnion but not the chorion is significantly affected by the chemical and mechanical changes during gestation and the labor process. These data will help direct future studies on the effects of clinical and molecular modulators of inflammation on membrane rupture thresholds with special emphasis on the biochemical and structural changes in the amnion.