We prospectively recorded placental site, maternal weight and parity in 182 patients reporting decreased fetal movements and compared these with controls. Reports of decreased fetal movements were 2.10 times (95% CI 1.51-2.92) as likely in association with an anterior placenta and 1.81 times (95% CI 1.20-2.93) and 2.61 times (95% CI 1.26-5.40) as likely in mothers weighing over 80 and 90 kg, respectively, but there was no association with parity. Decreased perception of fetal movements is more likely to have serious implications in thin women with a posterior placenta. Our findings also suggest that the sensation of fetal movements arises from pressure against body wall structures rather than the uterus or peritoneum.