Objective: A simplified geometric model of the uterine wall during the second and third stages of labor was created to estimate the magnitude of myometrial strain associated with the initiation of placental separation.
Study design: The uterine wall was modeled as an isovolumetric, incompressible spherical shell whose overall radius decreased and mural thickness increased 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 uterine mural thickness of 14 healthy patients from just before delivery to the time of initial separation of the placenta. The measured change in uterine wall thickness was then used to calculate its average radial and circumferential strain with a simple mathematic model.
Results: Placental separation occurred at radial and circumferential strains (mean +/- SD) of 450% +/- 182% and -75% +/- 11%, respectively. These strains are consistent with the known maximal contractile strains achievable by smooth muscle.
Conclusion: Placental separation is likely associated with maximal myometrial contractile strain. Before birth the presence of the fetal and amniotic fluid volumes usually renders such contractile strains unachievable, thereby helping to guard against premature placental separation.