Severe maternal morbidity in patients with pre-existing diseases: a prospective observational cohort study (ForMaT trial).

Severe maternal morbidity (SMM) is increasingly used as an indicator of maternal health. Registry-based studies have reported higher SMM rates among women with pre-existing diseases, but prospective data describing the timing and clinical context of these events remain limited. In this prospective observational cohort study conducted at a tertiary perinatal center in Germany between November 2022 and March 2024, SMM was defined according to the Centers for Disease Control and Prevention (CDC) criteria. Outcomes in women with pre-existing diseases were compared with two control groups: healthy women with obstetric risk factors and women with uncomplicated pregnancies. A total of 657 women with pre-existing diseases and 742 women in the control groups were included in the final analysis. Overall, 19 women experienced SMM, including 14 in the study group and 5 in the control groups. Women with pre-existing diseases had a higher risk of SMM than controls (relative risk (RR) 3.40, 95% confidence interval 1.22-9.49, adjusted for age, body mass index and parity). Half of the SMM events in the study group occurred antepartum and were attributable to the underlying disease, whereas events in the control groups occurred predominantly during the peripartum period and were mainly obstetric in origin. Women with pre-existing diseases also had higher risks of preterm birth (RR = 2.27; 95% CI 1.64-3.44) and neonatal intensive care unit admission (RR = 1.84; 95% CI 1.31-2.59). These findings suggest that pre-existing diseases substantially increase the risk of severe maternal morbidity and that a considerable proportion of events occur before delivery and are related to the underlying disease.