"Only a few previous studies have investigated the association between exercise and miscarriage. The only study that clearly supports our findings is a cohort study, which concluded that self-reported physical strain around the time of implantation (days 6–9 after ovulation) was associated with an increased risk of miscarriage (HR 2.5, 95% CI = 1.3–4.6). In contrast, Latka et al found a reduced risk of miscarriage with no chromosome defect in women who exercised compared with those who did not (OR = 0.5, 95% CI = 0.3–1.0). The case–control design was, however, based on a hypothesis that exercise cannot lead to chromosome aberrations in the fetus, as the control group consisted of women with miscarriages with chromosome aberrations. This assumption may be questioned since mode of action is unknown. In a small prospective study, Clapp found no statistically significant difference in risk of miscarriage between recreational runners (n = 49), aerobic dancers (n = 39) and a control group of active women, who had stopped exercising before the time of conception (n = 29). The study population was in excellent condition and had been exercising for years prior to the pregnancy, and the results may not be representative of the population at large. Two other studies have only investigated late miscarriages."