Objective The aim of this study was to identify the social, biological, behavioural and psychological factors related to physical activity (PA) in early pregnancy.Design This is a secondary analysis of data from a prospective cohort study.Setting The study was conducted in Cork, Ireland.Participants Nulliparous women with singleton pregnancies were recruited and then interviewed at 15 +/- 1 weeks' gestation.Primary and secondary outcomes The biopsychosocial model identified factors including social (age), biological (body mass index), behavioural (diet) and psychological (anxiety) at 15 +/- 1 weeks' gestation. PA subgroups were identified based on a latent class analysis of their responses to a set of questions about the amount and intensity of activity they were engaging in during the pregnancy. Associations were estimated with multivariable multinomial logistic regression models.Results From a total of 2579, 1774 (69%) women were recruited; ages ranged from 17 to 45 years. Based on a combination of model fit, theoretical interpretability and classification quality, the latent class analyses identified three PA subgroups: low PA (n=393), moderate PA (n=960) and high PA (n=413). The fully adjusted model suggests non-smokers, and consumers of fruit and vegetables were more likely to be in the high PA subgroup (vs low). Women with more than 12 years of schooling and a higher socioeconomic status were more likely to be in the moderate PA subgroup (vs low).Conclusion The findings highlight potential links between PA, a low education level and a low socioeconomic background. These factors should he considered for future interventions to improve low PA levels during pregnancy.