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.