Strengths and limitations
This RCT is to the best of our knowledge the largest study in this field. Compliance with the intervention was registered by the physiotherapists to ensure accuracy of the data and this is also considered a strength. The degree of exertion during exercise sessions was subjectively measured using The Borg Scale of Percived Exertion (26). The use of accelerometers would have made it possible to objectively determine the actual time and intensity (27). It is a strength that WHO-5 is a simple, validated patient-reported outcome measure, used as an outcome in the obstetric field (22), although not validated in a population of pregnant women. The generalisability of the trial results is limited due to a selected population of women at high risk for perinatal depression where the participants were highly educated, had a high level of physical activity before, during and after pregnancy, had normal BMIs, and understood the Danish language. It might be seen as a limitation that a large proportion of eligible women declined to participate. Based on the legislation, The Ethics Committee of the capital region of Denmark could not to let us describe the characteristics of those who declined to participate. We cannot rule out that there is a greater proportion of multi parous among the women who declined to participate. In this study population approximately 25% was multi parous, and in the population of pregnant women referred to Rigshospitalet, the proportion of multi parous is approximately 40%. One of the inclusion criteria was depression and/or anxiety requiring treatment by a psychiatrist, general practitioner or a psychologist within the last ten years before pregnancy. This information was self-reported, which is considered a strength in the context, since mild to moderate mental disorders are often diagnosed and treated outside primary care and thus not registered in the medical records nor in The Danish Psychiatric Central Research Register (28). To strengthen the validity of self-reported diagnoses, the pregnant women were asked to elaborate on the circumstances related to the diagnosis, including symptoms and treatment, when contacted by telephone by a specialised midwife.