Myometrial contractile strain at uteroplacental separation during parturition

OBJECTIVE: A simplified geometric model of the uterine wall during the seco nd and third stages of labor was created to estimate the magnitude of myome trial strain associated with the initiation of placental separation. STUDY DESIGN: The uterine wall was modeled as an isovolumetric, incompressi ble spherical shell whose overall radius decreased and mural thickness incr eased on uterine muscle contraction after delivery of the fetus. Either a 3 .5-MHz or a 5-MHz ultrasonography probe was used to measure the change in u terine mural thickness of 14 healthy patients from just before delivery to the time of initial separation of the placenta. The measured change in uter ine wall thickness was then used to calculate its average radial and circum ferential strain with a simple mathematic model. RESULTS: Placental separation occurred at radial and circumferential strain s (mean It SD) of 450% +/- 182% and -75% +/- 11%, respectively. These strai ns are consistent with the known maximal contractile strains achievable by smooth muscle. CONCLUSION: Placental separation is likely associated with maximal myometri al contractile strain. Before birth the presence of the fetal and amniotic fluid volumes usually renders such contractile strains unachievable, thereb y helping to guard against premature placental separation.