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.