FECUNDABILITY OF FEMALE TWINS

Animal studies have shown evidence of prenatal hormonal interaction be tween unlike sexed fetuses, including reduced fertility among females. We evaluated whether the fecundability of female twins is different f rom that of singletons and whether it differs according to the sex of the co-twin. The study was based on a questionnaire survey of 12,681 f emale twins born in the period 1953-1976 and an interview survey of 76 0 female controls born in the period 1953-1966, both in Denmark. Outco me of the first try ever to become pregnant (pregnant, still trying, s topped trying, pregnant despite contraception, and never tried) and th e waiting time to pregnancy distribution did not differ among monozygo tic, dizygotic same sexed, and dizygotic unlike sexed twins. More twin s had a waiting time of less than 2 months, compared with singletons. This difference probably reflects an artifact due to the data collecti on method, because it disappeared when the cutoff point was changed to include 2 months for singletons, and we found no difference for longe r waiting times. Hence, we found no increase in fecundability for twin s compared with singletons, nor any reduced fecundability among female twins from unlike sexed pairs.