BACKGROUND:We conducted a cohort study in an attempt to determine whether prolonged pregnancy in mother is a risk factor for prolonged pregnancy in daughter, and if previous prolonged pregnancy is a risk factor for prolonged pregnancy in subsequent pregnancy. METHODS:Data from the Swedish Medical Birth Registry were combined with a local registry of births (1955-1990). Mother-daughter pairs (with events of delivery in each generation) were identified. Relative risk (RR) and its 95% confidence interval (CI) were calculated and population attributable proportion was estimated when appropriate. RESULTS:If mother had had prolonged pregnancy at delivery of daughter the relative risk (RR) of prolonged pregnancy in daughter was moderately raised (RR = 1.3; CI : 1.0-1.7) with population attributable proportions ranging between 2.1% and 4.6%. If previous pregnancy had been prolonged, the RR of prolonged pregnancy at subsequent birth was increased 2-3 fold with population attributable proportions of 12.5% to 15.8%. Possible confounders such as mother's parity, age and maternal age did not alter the risks. CONCLUSIONS:Although moderate, prolonged pregnancy in mother may be a risk factor for prolonged pregnancy in daughter. A previous prolonged pregnancy increases the risk of prolonged pregnancy in a subsequent birth. However, the familial factor of prolonged pregnancy explains just a minor part of its occurrence in the population (due to small population attributable proportions).