Perinatal epidemiology provides the clinician working in antenatal care information about risk factors for adverse pregnancy outcome. However, many studies do not take account of dependencies between individuals in the study populations, for instance between siblings. Such dependencies may modify risk to a considerable degree. At least half of the appointments in antenatal care are with women who have previously been pregnant and given birth. The information these mothers can provide through their previous experiences is of great importance for the clinician when assessing the present pregnancy.