We have studied the characteristics of conceptional and pre-conceptional cycles in mothers of malformed infants. A comparison made with a control group of mothers of normal, term infants showed that for the former the hypothermic phase during the conceptional cycle was longer than for the latter (20.4 and 16.9 days, respectively). Moreover, the mothers of malformed infants showed a slower temperature rise (greater than 3 days) in 45% of cases, vs 28% in the controls. The menarche of the mothers of malformed infants occurs later (14.3 vs 12.8 years). They usually have long menstrual cycles and a bad obstetric and gynaecological history. The risk for congenital malformations is thus closely related to the length of the hypothermic phase and to a slow temperature rise in the conceptional cycle. Therefore we suggest that the preovulatory oocyte overripeness is one of the mechanisms of congenital malformation.