Introduction
Prediction of severe maternal morbidity has been identified as a critical research gap in obstetrics.1 Industrialized countries such as Canada and the UK experience similar low levels of maternal mortality, necessitating a shift in focus on ‘near miss’ events as a means to improving the health and quality of care for pregnant women.2 Many maternal characteristics are known pre-conception or early in pregnancy and are strong risk factors for the development of severe maternal morbidity.3,4Therefore, a combination of such factors may reliably predict its onset, enabling evidence-based and rational early triage of high-risk women for enhanced surveillance and subspecialty-based care.
Advances in maternal morbidity risk prediction include a US obstetric comorbidity index,5 which was externally validated within a Canadian population, resulting in modest discrimination (C-statistic of 0.66, 95% confidence interval [CI] 0.65-0.67).6 That index included variables that both preceded, and were simultaneous with, the onset of severe maternal morbidity, making it a useful research tool for identifying the burden of morbidity but less so for clinical prediction. Others have developed models focused on specific subtypes of maternal morbidity, such as cardiovascular-related conditions.7 Models predicting maternal mortality include the Collaborative Integrated Pregnancy High-dependency Estimate of Risk (CIPHER) model (C-statistic 0.82, 95% CI 0.81-0.84) and the Maternal Severity Index (C-statistic 0.83, 95% CI 0.80-0.85),8 both developed among women either already critically ill or hospitalized, and mostly later in gestation.
Since severe maternal morbidity predominantly arises around birth or early postpartum,9 the ideal timeframe for prediction is before or early in pregnancy to facilitate effective preventive strategies such as referral to high-risk centres or shared-care antenatal care pathways.10,11 Existing models do not enable these latter steps, nor do they account for important pre-pregnancy factors, such as maternal infertility and its treatment, which are associated with severe maternal morbidity.12Additionally, existing prediction efforts did not consider prior adverse pregnancy outcomes among parous women. We therefore undertook the current study to develop and internally validate a clinical prediction model (CPM) of severe maternal morbidity, defined as acute end-organ injury or death, using readily available factors ascertained pre-pregnancy and prior to 20 weeks’ gestation in a population-based study in Ontario - Canada’s most populous and multi-ethnic province.