Length of the follicular phase, time of insemination, coital rate and the sex of offspring.

The penetrability of cervical mucus improves over the follicular phase. When the length of the follicular phase varies due to variation in the timing of the luteinizing hormone surge, mucus penetrability will also improve as the phase lengthens. As selection for Y spermatozoa decreases with improvements in mucus penetrability, sex ratios at conception should decline in longer follicular phases. Sex ratios should also decline as the time of insemination approaches ovulation unless hormonally-induced improvements in penetrability are reduced by the debris left by earlier inseminations.