Although the first post-partum cycle is sometimes anovulatory (reducing the likelihood of becoming pregnant again before having a post-partum period), subsequent cycles are almost always ovulatory and therefore must be considered fertile. For women exclusively breastfeeding ovulation tends to return after their first menses after the 56 days postpartum time period. Supplementing nutritional intake can lead to an earlier return of menses and ovulation than exclusive breastfeeding. Nursing more frequently for a shorter amount of time was shown to be more successful in prolonging amenorrhea than nursing longer but less frequently. The continuing of breastfeeding, while introducing solids after 6 months, to 12 months were shown to have an efficiency rate of 92.6 – 96.3 percent in pregnancy prevention. Because of this some women find that breastfeeding interferes with fertility even after ovulation has resumed.