Association of birth outcome with subsequent fertility.

The association between birth outcome and subsequent fertility was analyzed by using linked Norwegian birth certificates. All births of order 1, 2, and 3 which occurred during 1967 through 1974 were considered index births; there were approximately 207,000 index births of order 1, 165,000 of order 2, and 87,000 of order 3. The mothers' fertility after these index births was summarized with a life-table technique. Fertility was most pronounced if there were no survivors of an index birth, intermediate if there was one survivor, and lowest if both members of a set of twins survived. Advanced maternal age was associated with markedly reduced fertility. The sex of a surviving singleton had little effect on a mother's subsequent fertility. However, there was a sex-related difference if index twins survived; fertility was lower after the birth of unlike-sex twins and higher after the birth of like-sex twins. This probably reflects reproductive limitation rather than a differential fecundity for mothers of dizygotic and monozygotic twins. A comparison of fertility after births of like-sex and unlike-sex twins with one survivor may indicate that mothers of dizygotic twins were more fertile, but the number available for study was small. Reproduction among women who had two index births during 1967 through 1974 was examined separately. Fertility was most marked if neither of the first two infants survived and lowest where three survived (i.e., where one of the index births involved twins). If there were two survivors, the sex composition of the pair influenced fertility; fertility was greater if the two survivors were of the same sex and lower if they were of unlike sex. Since a woman who has an unfavorable outcome in one pregnancy will be at a higher risk of having an unfavorable outcome in a subsequent pregnancy, the higher fertility of such women will, to some degree, inflate the frequency of unfavorable outcomes in a population of births.