BACKGROUND: Successful introduction, high uptake and program effectiveness of new maternal vaccines aimed to prevent disease among infants require prior knowledge of their safety during pregnancy. We aimed to identify background adverse birth outcomes and their predictors in Kenya by which to aid future interpretation of outcomes for new maternal vaccination programs. METHODS: A cross-sectional survey was conducted to assess birth outcomes from women residents within the health and demographic surveillance systems of Kilifi, Siaya and Nairobi, Kenya. All selected women had pregnancies registered in the years 2017-2020 through census rounds and had a birth outcome recorded by the time of data collection. They were traced at home for interviews and abstraction of birth outcome records from mother and child health booklets. Multivariable logistic regression was used to identify independent predictors of adverse birth outcomes. RESULTS: A total of 2702 women were interviewed. Adverse birth outcomes occurred in 788/2702 (29.2%) of pregnancies: 433 (16.0%) were preterm (gestational age <
37 weeks), 298 (11.0%) low birth weight (<
2500 g), 99 (3.7%) macrosomic (>
4000 g) and 41 (1.5%) stillbirths. Predictors of adverse birth outcomes were gestational diabetes [adjusted Odds Ratio (aOR): 3.32 (1.53-7.20)], malaria during pregnancy [aOR: 1.74 (1.23-2.48)], not attending antenatal care [aOR: 12.89 (2.17-76.68)] and home delivery [aOR: 1.58 (1.18-2.12)]. CONCLUSIONS: In 3 Kenyan settings, almost a third of pregnancies had adverse birth outcomes. Recognizing this baseline prevalence and the factors associated with adverse birth outcomes will be important in validating the safety of new maternal vaccines.