Is recreational running associated with earlier delivery and lower birth weight in women who continue to run during pregnancy? An international retrospective cohort study of running habits of 1293 female runners during pregnancy

Background Increasingly, women of reproductive age participate in recreational running, but its impact on pregnancy outcome is unknown. We investigated whether running affects gestational age at delivery and birth weight as indicators of cervical integrity and placental function, respectively. Methods 1293 female participants were recruited from parkrun, which organises weekly runs involving 1.25 million runners across 450 parks worldwide. Those under 16 or unable to provide outcome data were excluded. Women were categorised according to whether they continued to run during pregnancy or not. Those who continued were further stratified dependent on average weekly kilometres, and which trimester they ran until. Retrospectively collected primary outcomes were gestational age at delivery and birthweight centile. Other outcomes included assisted vaginal delivery rate and prematurity at clinically important gestations. Results There was no significant difference in gestational age at delivery: 279.0 vs 279.6 days (mean difference 0.6 days, CI -1.3 to 2.4 days; P=0.55) or birthweight centile: 46.9%vs 44.9% (mean difference 2.0%, CI -1.3% to -5.3%; P=0.22) in women who stopped running and those who continued, respectively. Assisted vaginal delivery rate was increased in women who ran: 195/714 (27%) vs 128/579 (22%) (OR 1.32; CI 1.02 to 1.71; P=0.03). Conclusion Continuing to run during pregnancy does not appear to affect gestational age or birthweight centile, regardless of mean weekly distance or stage of pregnancy. Assisted vaginal delivery rates were higher in women who ran, possibly due to increased pelvic floor muscle tone. Randomised prospective analysis is necessary to further explore these findings.