Little is known about the cytoarchitecture of human myometrial cells in pregnancy, and whether or not this may be influenced by maternal characteristics such as age, parity and body mass index (BMI). The aim of this study was primarily to evaluate human myometrial smooth muscle cell (SMC) and nuclear volume in the third trimester of human pregnancy, and secondarily to investigate if these parameters are altered in relation to the maternal characteristics outlined above. Myometrial biopsies were obtained from 30 women undergoing elective caesarean delivery at term. One-micrometer sections were prepared for light microscopy and 100-nm sections for electron microscopy. The nucleator technique was used to assess nuclear volume from the light microscopy images. Point-counting methodology was used on transmission electron micrographs to assess the percentage of the cell volume occupied by the nucleus. Cell volume was calculated from these measurements. The euchromatin to heterochromatin (Eu/Het) ratio was determined to ascertain whether differences in nuclear volume were due to an increased range of genes being transcribed. The mean (± SEM) nuclear volume was 175 ± 10 μm(3) , the nucleus occupied 1.5 ± 0.1% of the SMC and the mean cell size was 14 047 ± 1352 μm(3) . The Eu/Het ratio was 7.54 ± 0.4. The mean volume of heterochromatin and euchromatin in the nucleus was 21.5 ± 1.7 and 149 ± 9 μm(3) , respectively. A multivariate regression analysis revealed that advanced maternal age was associated with an increase in the percentage of the cell occupied by nucleus (R(2) = 0.32, P = 0.004). There were no other significant effects of maternal age, BMI or parity on the measured parameters. These findings provide reliable volumes for human myometrial cells and their nuclei at term gestation, and show that nuclear volume fraction may be influenced by maternal age.