or digital intercourse) involves the manual manipulation of the clitoris, rest of the vulva, vagina or anus for the purpose of sexual arousal and sexual stimulation; it may constitute the entire sexual encounter or it may be part of mutual masturbation, foreplay or other sexual activities. Reproduction Natural human reproduction involves penile–vaginal penetration, during which semen, containing male gametes known as sperm cells or spermatozoa, is expelled via ejaculation through the penis into the vagina. The sperm passes through the vaginal vault, cervix and into the uterus, and then into the fallopian tubes. Millions of sperm are present in each ejaculation to increase the chances of fertilization (see sperm competition), but only one reaching an egg or ovum is sufficient to achieve fertilization. When a fertile ovum from the female is present in the fallopian tubes, the male gamete joins with the ovum, resulting in fertilization and the formation of a new embryo. When a fertilized ovum reaches the uterus, it becomes implanted in the lining of the uterus (the endometrium) and a pregnancy begins. Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some 5 days before until 1 to 2 days after ovulation. For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days, or every 2 or 3 days. Studies have shown no significant difference between different sex positions and pregnancy rate, as long as it results in ejaculation into the vagina. When a sperm donor has sexual intercourse with a woman who is not his partner and for the sole purpose of impregnating the woman, this may be known as natural insemination, as opposed to artificial insemination. Artificial insemination is a form of assisted reproductive technology, which are methods used to achieve pregnancy by artificial or partially artificial means. For artificial insemination, sperm donors may donate their sperm through a sperm bank, and the insemination is performed with the express intention of attempting to impregnate the female; to this extent, its purpose is the medical equivalent of sexual intercourse. Reproductive methods also extend to gay and lesbian couples. For gay male pairings, there is the option of surrogate pregnancy; for lesbian couples, there is donor insemination in addition to choosing surrogate pregnancy. Safe sex and birth control There are a variety of safe sex methods that are practiced by heterosexual and same-sex couples, including non-penetrative sex acts, and