Masturbation may increase fertility during intercourse.
Female masturbation alters conditions in the vagina, cervix and uterus, in ways that can alter the chances of conception from intercourse, depending on the timing of the masturbation. A woman's orgasm between one minute before and up to 45 minutes after insemination favors the chances of that sperm reaching her egg. If, for example, she has had intercourse with more than one male, such an orgasm can increase the likelihood of a pregnancy by one of them. Female masturbation can also provide protection against cervical infections by increasing the acidity of the cervical mucus and by moving debris out of the cervix.
In males, masturbation flushes out old sperm with low motility from the male's genital tract. The next ejaculate then contains more fresh sperm, which have higher chances of achieving conception during intercourse. If more than one male has intercourse with a female, the sperm with the highest motility will compete more effectively.