Parents of autistic children report barriers to engaging in physical activity, which may be exacerbated during subsequent pregnancies. We aimed to describe physical activity of parents caring for an autistic child, before and during a subsequent pregnancy, and to explore whether physical activity was associated with the autistic child's Social Responsiveness Scale score, a measure of autism-related traits. We used data from the Early Autism Risk Longitudinal Investigation, in which families with an autistic child were followed through a subsequent pregnancy. Mothers (n = 245) self-reported physical activity in the 3 months before conception and during pregnancy; fathers (n = 130) reported on the 6 months prior to enrollment. Approximately 40% of nonpregnant mothers and fathers and 9.3% of pregnant mothers met Physical Activity Guidelines for Americans recommendations. Most (83.5%) pregnant mothers reported no vigorous activity; after adjustment for covariates, this was more common among mothers of children with Social Responsiveness Scale T-scores >75 compared with mothers of children with lower T-scores (adjusted odds ratio (95% confidence interval) = 2.94 (1.11, 7.78)). Among parents caring for an autistic child before and during a subsequent pregnancy, physical activity was lower than recommended. Family-based interventions may be necessary to help support physical activity levels.