OBJECTIVE: Knowledge of the stereology of human myometrium in pregnancy is limited. Uterine contractile performance may be altered in association with maternal obesity and advanced maternal age. The aim of this study was to investigate the stereology of human myometrium in pregnancy, and to evaluate a potential influence of maternal body mass index (BMI) and age.STUDY DESIGN: Biopsies of human myometrium were obtained from 57 women at cesarean section (n = 26, n = 13, n = 18 normal, overweight and obese BMI, respectively), and volume fractions of smooth muscle and extracellular matrix were assessed using stereologic techniques.RESULTS: The smooth muscle constituted 65.2% +/- 8.9% (standard deviation) and the extracellular matrix 32.6% +/- 7.7% (standard deviation) (n = 57). There was no correlation observed between maternal BMI, age, or parity with the fractional volumes of either smooth muscle or extracellular matrix.CONCLUSION: These results outline the stereology of human myometrium in pregnancy. Putative functional differences in contractility, pertaining to obese or older mothers, are not related to smooth muscle content.