[Responses to a postal survey on women's health after birth: importance of repeated mailings and characteristics of non-respondents].

A longitudinal survey was carried out in three maternity units in France in 1993-1994. At the first contact, within the four days following the delivery, 761 women were included in the survey with a face-to-face interview. The second and the third contacts took place five and twelve months after the birth. These contacts were postal questionnaires at home, with a first mail followed by three reminders, if necessary. This paper analyses the response rate to a postal survey and the efficiency of repeated mails in the context of a research about women's health after childbirth. The characteristics of non responders and those of late responders are compared with initial stage responders. Twelve months after the birth, 57% of the women had returned the questionnaire after the first mail and 86% after all the mailings. Non responders were younger and were more often in unfavorable family or social conditions than responders. The late responders had more often a non French nationality, they reported more often psychotropic drugs consumption and wished for this pregnancy less often than the initial stage responders. For some characteristics as educational level, employment or financial concerns late responders had an intermediate position between the initial stage responders and the non responders. Repeated mailings strongly improve the final response rate. Nevertheless non responders remain a particular group that is imperfectly represented by the late responders.