OBJECTIVES: To synthesise evidence on the association between any diagnosed or self-reported mental health problems prior to pregnancy (pre-existing mental health problems) and birth outcomes including preterm birth (PTB), low birth weight (LBW), small for gestational age (SGA), neonatal unit (NNU) admission and mode of birth (instrumental birth, planned or unplanned caesarean section). METHODS: Systematic searches were conducted in MEDLINE, CINAHL, Embase and PsycINFO in December 2024 for studies examining the association between any pre-existing mental health problems and PTB, LBW, SGA, NNU admission and mode of birth. Only articles published in English were included with no restriction on year of publication. Two reviewers independently screened studies and extracted data. Study quality was assessed using the Newcastle-Ottawa Scale and Joanna Briggs Institute checklists. Random-effects meta-analyses were conducted to pool crude and adjusted ORs (aORs) and risk ratios (aRR) with 95% CIs. ORs and RRs were analysed separately. Between-study heterogeneity was quantified using the I2 statistic. RESULTS: Of 15 467 records screened, 33 studies met the inclusion criteria. Women with any pre-existing mental health problems had higher odds and risks of adverse birth outcomes, including PTB (aOR 1.41, 95% CI 1.27 to 1.56) (aRR 1.36, 95% CI 1.21 to 1.51), LBW (aOR 1.28, 95% CI 1.22 to 1.33) (aRR 1.32, 95% CI 1.04 to 1.68), SGA (aOR 1.27, 95% CI 1.07 to 1.51) (aRR 1.34, 95% CI 1.19 to 1.51) and NNU admission (aOR 1.44, 95% CI 1.19 to 1.74). Adjusted estimates were based on multivariable models that commonly controlled for maternal age, parity and socio-demographic factors. No consistent associations were observed between pre-existing mental health problems and mode of birth. CONCLUSIONS: Pre-existing mental health problems were associated with increased risks and odds of several adverse birth outcomes. These findings highlight the importance of early identification and targeted support for women with mental health problems before pregnancy to strengthen preconception and maternity care planning. PROSPERO REGISTRATION NUMBER: CRD42023485834.