Prevalence of postdates pregnancy worldwide is 5-10% but the aetiology remains unknown. We have shown that cholesterol inhibits myometrial contractility in vitro. We examined the contractile abilities of myometrium from postdates women and investigated if differences in myometrial plasmalemmal cholesterol content may be contributing to prolonged gestation by increasing myometrial quiescence. Following informed consent, myometrial biopsies were obtained from women undergoing emergency caesarean section in labour (n=67) at different gestations. Dissected longitudinal strips were placed in a small (0.2ml) tissue bath and superfused with physiological saline. Integral force of spontaneous activity in 20min and under 10nM oxytocin was compared between gestational groups. In some tissues simultaneous recording of Ca signalling was also performed (INDO-1 fluorescence). 5mg blotted weight strips of myometrium were dissected for chloroform/methanol lipid and cholesterol extraction. Membrane cholesterol content was determined by thin layer chromatography. The ratio of cholesterol to phosphatidylethanolamine (PE) internal control was used to assess the relationship between cholesterol and gestational age by Pearson’s correlation (p<0.05 being significant). Cholesterol:PE ratios were normally distributed (mean 0.23 ± 0.103µg cholesterol/mg myometrium). Patients were sub-grouped into LOW (n=11) NORMAL (n=47) and HIGH (n=9) cholesterol based upon values less than and greater than one SD either side of the mean (LOW and HIGH groups respectively. Mean (± S.E.M) gestational age per group was then calculated and compared. Contractility of postdates myometrium was less forceful and showed significantly reduced integral force compared to term controls (8.65a.u.(n=22) verses 3.72a.u.(n=15) respectively, p=0.013 Student’s t-test) and the accompanying Ca transients were also reduced. Integral force under oxytocin was also lower in the prolonged pregnancy group. We find a positive trend between increasing gestational age and myometrial cell cholesterol. The LOW cholesterol group had a lower gestation of 278days (± 5.82) compared to the NORMAL cholesterol group (282.5 days ± 2.11) whilst the HIGH cholesterol group has an even greater mean gestation (287.3days ± 1.89). Myometrium from women in the postdates group had higher mean CHOL:PE ratio (0.238 ± 0.02) compared to term controls (0.218 ± 0.02). Our in vitro data shows that uterine contractility is reduced in postdates pregnancies. Whilst differences in membrane cholesterol are small, our data indicate that elevated myometrial cell membrane cholesterol may be responsible for the reduced contractility observed and hence underlie the biological cause of prolonged gestation. To further explore the underlying mechanisms, changes in expression of contraction associated proteins are being determined.