[The critical period in natural fertility].

In order to learn the natural expectancy period of first pregnancy as well as the influences of some biosocial variables, a clinical-retrospective study was performed within 1120 gravida I women. All of them attended the major institutes of health care in Mexico City (IMSS, ISSSTE, SS and DDF). Data were obtained by means of predetermined questions from subjects after the first postpartum hours. None of them had previously used any contraceptive method. Results showed age at the first pregnancy: 21.6 +/- 3.8 (years); gestational interval 22.1 +/- 19.9 (months) and a 67% achievement rate the following twelve months of sexual activity. Eutocia was seen in 54% of these population and significant linear correlation was attained between age of the patient and the gestational interval. It seems that this is the first information regarding the fertility critical period within our society.