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Revisiting Severe Maternal Morbidity Recurrence: Selection Bias and the Implications for Risk Estimation Across
Rashida S Smith-Webb1, Martha M Werler, Samantha E Parker
1Department of Epidemiology, Boston University School of Public Health, Boston, MA 02118, USA.
Epidemiology (Cambridge, Mass.)
|July 21, 2026
Summary
Selective attrition minimally impacted severe maternal morbidity (SMM) recurrence estimates. This study found that accounting for participants who dropped out did not significantly alter the risk of SMM recurrence after childbirth.
Area of Science:
- Maternal Health
- Epidemiology
- Biostatistics
Background:
- Selective attrition can introduce bias in studies estimating severe maternal morbidity (SMM) recurrence.
- Previous research has not quantified the extent of bias caused by selective attrition in SMM recurrence.
- This study addresses the gap by examining bias in the association between SMM at index delivery and SMM recurrence at a second delivery.
Purpose of the Study:
- To quantify the bias introduced by selective attrition in the estimation of severe maternal morbidity (SMM) recurrence.
- To assess the impact of selective attrition on the association between SMM at index delivery and SMM risk at a subsequent delivery.
Main Methods:
- Utilized data from the Pregnancy to Early Life Longitudinal Data System in Massachusetts (1998-2021).
- Included nulliparous individuals with singleton, in-hospital deliveries, followed from index delivery to a second delivery or study end.
- Employed inverse probability (IP) of continuation weights, derived from logistic regression modeling attrition, to adjust estimates.
Main Results:
- Unweighted analyses showed SMM at index delivery associated with increased SMM recurrence risk (RD: 506.1; RR: 5.39).
- IP weighting minimally altered estimates (RD: 510.6; RR: 5.42), indicating <1% downward bias from attrition.
- Black individuals exhibited the highest SMM recurrence rates (RD: 1076.6; RR: 6.24).
Conclusions:
- Selective attrition demonstrated minimal influence on the estimated risk of severe maternal morbidity recurrence in this population.
- The findings suggest that existing estimates of SMM recurrence are largely robust to attrition bias in this specific setting.
- Highlights disparities in SMM recurrence, with Black individuals experiencing disproportionately higher rates.
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