Animal studies have shown evidence of prenatal hormonal interaction between unlike sexed fetuses, including reduced fertility among females. We evaluated whether the fecundability of female twins is different from 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 female twins born in the period 1953-1976 and an interview survey of 760 female controls born in the period 1953-1966, both in Denmark. Outcome of the first try ever to become pregnant (pregnant, still trying, stopped trying, pregnant despite contraception, and never tried) and the waiting time to pregnancy distribution did not differ among monozygotic, dizygotic same sexed, and dizygotic unlike sexed twins. More twins had a waiting time of less than 2 months, compared with singletons. This difference probably reflects an artifact due to the data collection method, because it disappeared when the cutoff point was changed to include 2 months for singletons, and we found no difference for longer waiting times. Hence, we found no increase in fecundability for twins compared with singletons, nor any reduced fecundability among female twins from unlike sexed pairs.