section_path: Behaviors/Reproduction
section_name: Reproduction
paragraph_text: Reproduction
  
  Natural human reproduction involves penile–vaginal penetration, during which semen, containing male gametes known as sperm cells or spermatozoa, is ejaculated 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 ejaculated to increase the chances of fertilization (see sperm competition), but only one is sufficient to fertilize an egg or ovum. When a fertile ovum from the female is present in the fallopian tubes, the male gamete fertilizes the ovum, forming a new embryo. Pregnancy begins after the fertilized ovum is implanted in the lining of the uterus (the endometrium).
  
  Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some five days before until approximately one day after ovulation (this is sometimes called the fertile window). For optimal pregnancy chance, there are recommendations of vaginal intercourse every one or two days, or every two or three days. Some people who are trying to conceive may choose to time vaginal intercourse with the fertile window, a practice that is sometimes called 'timed intercourse'. Timed intercourse using urine tests that predict ovulation may help improve the rate of pregnancy and live births for some couples trying to conceive such as those who have been trying for less than 12-months and who are under 40 years old, however, it is not clear from medical evidence if timed intercourse improves the rate of ultra-sound confirmed pregnancies and it is also not clear if timed intercourse has an effect on a person's level of stress or their quality of life. 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. ICI insemination, which involves the deposit of (usually) raw semen in the vagina of a woman, is in effect a substitute for intercourse and is often contrasted with 'normal intercourse' in this context. 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. Some women use artificial insemination to become single mothers by choice.