The concern in the past has been mainly that grand multiparae (mothers who have given birth five times before) were at higher risk of postpartum haemorrhage and some other complications, because the uterus might lose its elasticity and therefore might not contract down well after the birth.
The latest research and expert opinion suggests that in fact a woman who is healthy and who does not have a history of such complications is no more likely to be at risk simply because she is a grand multipara. Below you will find research abstracts, and a review from a professor of midwifery, which suggest that a woman's previous obstetric history and personal health are more relevant than the fact that she has a large family. The older studies which led to these worries may have been misleading because control groups were not matched for age or for pre-existing health problems such as diabetes or high blood pressure.