"In this study, based on data from nearly 93,000 women, a dose-response relation was seen for the association between the amount of weekly exercise and the risk of miscarriage early in pregnancy. Certain types of exercise, and particularly high-impact types of exercise, were found to be associated with a higher risk of miscarriage. In the analyses based only on prospectively collected exposure data, the association did, however, attenuate, indicating a certain degree of recall bias. An alternative explanation to recall bias may be that exercise only in the early stages of pregnancy hurts the pregnancy outcome. In this case, the difference in the HRs between the analyses based on the total data material and the subcohort of only prospectively collected data is not as much a result of the mode of data collection as a reflection of the fact that the total data material encompasses the very early miscarriages. Even within the subperiods of gestational age, the miscarriages occur earlier for the pregnancies with retrospectively collected exposure information than for pregnancies with a first pregnancy interview (Figure 2). In addition, we did see a positive trend in the association between exercise and the risk of miscarriages in the earliest period of pregnancy (gestational weeks 11–14) in the subcohort using prospectively collected data only."