Introduction The evaluation of severe maternal morbidity has been used in the monitoring of maternal health. The objective of this study is to estimate its incidence and main causes in São Luís-Maranhão, one of the poorest cities in Brazil.

Methods A prospective longitudinal study, carried out in two public high risk maternity clinics and two intensive care units (ICUs) of reference to obstetrical cases of the city. During the period of 1 March 2009 and 28 February 2010, all cases of severe maternal morbidity were identified following Mantel's and Waterstone's criteria. Socio-demographic characteristics and healthcare data of the extremely severe cases were compared to the less severe cases. The Fisher, χ², Student t, and Mann–Whitney tests were used for a statistical analysis, with significance levels of <0.05.

Results 127 cases of severe maternal morbidity among 8493 deliveries were identified, leading to an incidence of 15.0/1000 deliveries. 122 women were interviewed, five cases were lost (3.9%). 121 cases fell under Waterstone's criteria and 29 under Mantel's criteria, corresponding to incidences of 14.1/1000 and 3.4/1000 deliveries, respectively. These rates are lower than those described in the literature, possibly due to case loss. The main causes of morbidity were hypertension during pregnancy, more frequent in less severe cases (p=0.001) and obstetrical haemorrhage, more common among extreme severe cases (p=0.01).

Conclusion The obstetrical disorders are the main causes of severe maternal morbidity in São Luís/Maranhão. The investigation and monitoring of severe morbidity can contribute to improve the obstetrical assistance in the city.