FAQ: How Does Pregnancy Occur?

Detailed Question: 

How Does Pregnancy Occur?





The Basic Idea

There are four things that must happen for a woman to become pregnant. If all of these things happen, through any means, then the woman is pregnant:

  1.  Semen enters the vaginal canal
  2.  The sperm swim through the cervix and into the fallopian tubes
  3.  The sperm penetrates the surface of a egg and fertilizes it (this is called conception)
  4.  The fertilized egg travels to the uterus and implants into the uterine wall



Step 1: Sperm enter the vagina

*Note: please see the Anatomy Section for more information about the different parts of the male and female body.


  • If sperm enter the vagina, then they are in a position to complete the other three steps.
  • If they only reach the vulva (the outer genital area), they can still swim up to the vagina and continue on. However, this is much more difficult, so the chance of pregnancy is much smaller.
  • Cowper’s gland secretions (also known as precum) can also carry sperm, so precum can cause a pregnancy. This is one reason why the withdrawal method is not very effective.
  • It is possible (but unlikely) for sperm to travel through thin clothing (like underwear), but they cannot travel through thicker clothing (like jeans). Multiple layers of thin clothing (like underwear and pajama bottoms or both partners wearing underwear) also can stop sperm.
  • Sperm have a very difficult time traveling through water, so if the man ejaculates into the water of a bathtub or shower, away from the woman’s vulva, pregnancy is very unlikely. However, sexual intercourse in a pool or a bathtub is not an effective way to stop sperm. If the man ejaculates into the vagina or even on the vulva while in water, there chance of pregnancy is just as high as in the corresponding activity in dry air.
  • The best way to keep sperm from entering the vagina is to use a condom.


Step 2: Sperm swim through the cervix and uterus into the fallopian tubes.


  • Once sperm are in the vagina, they can swim through the cervix, uterus, and fallopian tubes. The more sperm that enter the vagina, the greater the chance of pregnancy.
  • Certain birth control methods interfere with the sperm’s ability to enter the cervix and continue on.
    • Hormonal methods like the pill or the shot change the cervical mucous, which makes it harder for the sperm to pass through the cervix.
    • Emergency contraception pills (also called the “morning-after pill” or Plan B) also interfere with the sperm entering the fallopian tubes.
    • Barrier methods like the diaphragm or the cervical cap create a physical obstruction, which is why they are called barrier methods.


Step 3: One sperm meets an egg and fertilizes it, which is called the moment of conception.


  • There has to be an egg in the outer third of a fallopian tube for the woman to become pregnant.
  • A woman has to ovulate shortly before or after intercourse to become pregnant.
    • Sperm can live for up to five days in the woman’s body, so if intercourse happens even a few days before ovulation, a sperm could still be present to fertilize the egg.
    • An egg can live for about one to two days, so intercourse after ovulation can still result in pregnancy.
  • A woman can ovulate before she has her first period, so intercourse before menarche (when the woman has her first period) can result in pregnancy.
  • Virginity has nothing to do with ovulation, thus a virgin can get pregnant her first time just as easily as her 100th time.
  • Many hormonal birth control methods like the pill or the shot inhibit ovulation, so an egg is not available to fertilize.
  • Emergency contraception pills also inhibit ovulation.


Step 4: The egg implants itself into the uterine wall, which is the start of pregnancy.


  • A fertilized egg travels down the fallopian tubes and may succeed in implanting into the uterine wall.
  • Many hormonal birth control methods like the pill or the shot change the uterine lining, making it inhospitable to the fertilized egg (meaning the conceptus won’t have a place to implant and grow).
  • Emergency contraception pills also make the uterine lining inhospitable to implant



UCSB SexInfo Copyright © 2016 University of California, Santa Barbara. All Rights Reserved.