For additional information, see Characteristics of included studies, Characteristics of excluded studies and Characteristics of ongoing studies.
Incomplete outcome data
As most of the data were collected by questionnaire, an attrition rate was reported. In the Ickovics 2007a trial, all participants completed the baseline interview. Eighty-nine completed the third trimester interview, 75% completed six-month follow-up and 80% completed 12-month follow-up. Medical record data were collected for 95% of randomly assigned women. In the Kennedy 2011 trial, 10% of women were lost to follow-up. It is possible that participants who were lost to follow-up in both studies were those with more negative views or experiences. Similarly, in the Andersson 2013 trial, attrition bias was possible, given that the second questionnaire at six months' postpartum was completed by 53.5% of women (228/426) in the group-based care group, and by 49.7% of women (179/360) in the individual care group. The Jafari 2010 trial reported small attrition rates of 2% in the group care group and 3.6% in the individual care group