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The lifespan of sperm is affected by many factors, which means it may survive anywhere from a few minutes to seven days. Sperm tends to die in extreme temperatures, so avoiding environments that are overly hot or cold can increase its lifespan. It also needs moisture to survive for more than an hour, because dryness can be deadly. Additionally, the type of chromosome carried by the sperm may influence its longevity, which is why knowing the life cycle of sperm may help couples who are trying to conceive.
While semen is in a man's body, it needs to be kept at the right temperature. In most cases, sperm has the longest lifespan when it is kept slightly colder than normal body temperature, which is why the testicles are positioned just outside the body inside the scrotum. Heat can kill sperm, which is why men are often advised to avoid hot baths, hot tubs and saunas if they want to increase the lifespan of sperm. The sperm also should not get too cold, either, which is why the scrotum tends to contract to force the testicles closer to the body when the temperature gets too low. After ejaculation, the sperm still needs to be kept at a healthy temperature, which is partly why it tends to live longer in a woman's body than outside it.
Sperm also requires moisture to live, so it tends to die within 20 minutes to an hour if it lands on the skin or another dry surface. It can, therefore, survive much longer once ejaculated into a woman's body, especially once it reaches the uterus. If the semen stops at the vagina, it can die within a few hours because of the acidic vaginal discharge that may be present, but most of the semen should continue into the cervix. The lifespan of sperm in the cervix is increased when the woman is ovulating, because the cervical mucus is copious, thin and easy to travel through at that time, but it tends to become thick at other times, trapping and killing sperm quickly. For the semen that reaches the uterus and Fallopian tubes, the lifespan of sperm is about five to seven days, because the moisture and overall environment can be considered just right.
Another factor that may affect the lifespan of sperm is the type of chromosome it has. Sperm carrying the Y chromosome will create a boy, while sperm with the X chromosome will usually result in a girl, which may interest couples with a gender preference. This is because the sperm carrying the Y chromosome tends to swim faster but also may be more sensitive to high acidity in the vagina and die sooner. Sperm carrying an X chromosome is usually slower but can withstand an acidic environment better. The vagina often becomes less acidic on the day of ovulation, so couples hoping for a boy may choose to have intercourse that day, while those who want a girl may have better luck with intercourse a few days before the woman ovulates, because the sperm should be able to live inside her for up to seven days.