Risk of preterm birth by subtype among Medi-Cal participants with mental illness

Background Previous studies have demonstrated an association between mental illness and preterm birth (before 37 weeks). However, these investigations have not simultaneously considered gestation of preterm birth, the indication (eg, spontaneous or medically indicated), and specific mental illness classifications. Objective The objective of the study was to examine the likelihood of preterm birth across gestational lengths and indications among Medi-Cal (California's Medicaid program) participants with a diagnostic code for mental illness. Mental illnesses were studied by specific illness classification. Study Design The study population was drawn from singleton live births in California from 2007 through 2011 in the birth cohort file maintained by the California Office of Statewide Health Planning and Development, which includes birth certificate and hospital discharge records. The sample was restricted to women with Medi-Cal coverage for prenatal care. Women with mental illness were identified using International Classification of Diseases, ninth revision, codes from their hospital discharge record. Women without a mental illness International Classification of Diseases, ninth revision, code were randomly selected at a 4:1 ratio. Adjusting for maternal characteristics and obstetric complications, relative risks and 95% confidence intervals were calculated for preterm birth comparing women with a mental illness diagnostic code with women without such a code. Results We identified 6198 women with a mental illness diagnostic code and selected 24,792 women with no such code. The risk of preterm birth in women with a mental illness were 1.2 times higher than women without a mental illness (adjusted relative risk, 1.2, 95% confidence interval, 1.1–1.3). Among the specific mental illnesses, schizophrenia, major depression, and personality disorders had the strongest associations with preterm birth (adjusted relative risks, 2.0, 2.0 and 3.3, respectively). Conclusion Women receiving prenatal care through California's low-income health insurance who had at least 1 mental illness diagnostic code were 1.2–3.3-times more likely to have a preterm birth than women without a mental illness, and these risks persisted across most illness classifications. Although it cannot be determined from these data whether specific treatments for mental illness contribute to the observed associations, elevated risk across different diagnoses suggests that some aspects of mental illness itself may confer risk.