A study of 4,000 pregnant women has found that obese women had an increased risk of a prolonged first stage of labour, and almost 1 in 5 women had to undergo an emergency Caesarean section because of the poor performance of their uterine muscles. The research suggests that obesity impairs the ability of the uterus to contract sufficiently to dilate the cervix and deliver the baby.
Obese women who gave birth vaginally were also found to encounter other problems in child birth - more than twice as many (6%) experienced excessive bleeding following delivery compared with normal weight women (3%). This blood loss was also attributed to poor uterine activity in the obese group which failed to clamp off the blood vessels that shear off after delivery of the placenta.
In the obese women, less calcium was able to enter the uterine cells to help muscles to contract during labour, perhaps due to the high levels of cholesterol in an obese woman's bloodstream. This could disrupt cell membranes and signalling pathways, including the entry of calcium.
Zhang G, Bricker L, Wray S, Quenby S. Poor uterine contractility in obese women. British Journal of Obstetrics and Gynaecology 2007; 114(3): 343-348. E-pub 2007