Introduction Maternal perinatal mental illnesses are crucial public health issues and mostly addressed in UK primary care. This study was to assess whether there were variations in the burden of mental illnesses by age and socioeconomic status (SES).

Methods We randomly selected one pregnancy ending in a live birth for women age 15–45 years from 1994 to 2009 from The Health Improvement Network, a nationally representative electronic primary healthcare database. We assessed the association of maternal perinatal depression, anxiety and severe mental illnesses (eg, schizophrenia) with maternal age and SES (measured by the Townsend Index) using logistic regression. We checked for evidence of effect modification between age and SES using a likelihood ratio test for interaction.

Results Compared with the highest socioeconomic group, women age 35–45 years in the lowest socioeconomic group were 2.8 times (95% CI 2.4 to 3.4) more likely to have depression during pregnancy and 2.4 times (95% CI 2.1 to 2.7) after pregnancy whereas women age 15–24 years were 1.4 times (95% CI 1.1 to 1.8) and 1.6 times (95% CI 1.4 to 1.8). Similar pattern was found for anxiety. After mutually adjusted for age and SES, older women or women with lower SES were also more likely to have severe mental illnesses.

Conclusion We find that women with lower SES are more likely to have perinatal mental illnesses. This is more evident in older than younger women for depression and anxiety. Perinatal healthcare professionals therefore should be aware of this and provide further effective interventions to this high-risk group.