How Deep Does Semen Need To Go To Cause Pregnancy?

Photographed by Nicolas Bloise.
This post was originally published on November 3, 2014.
How far do sperm have to go in a vagina for someone to get pregnant?
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UPDATE: Based on readers' comments, we wanted to jump in here and explain that, for starters, although the question that came to us asked about sperm, what we're really addressing here is semen.
As it states in the article, semen (cum) is the fluid you can see that comes out of a penis during ejaculation. Sperm are the microscopic cells swimming inside the semen that cause pregnancy. So yes, sperm cells themselves must go "deep" in the vagina, through the cervix and uterus, to the fallopian tubes, and meet with an egg to cause pregnancy. But, sperm cells can make that journey even if SEMEN isn't placed "deep" in the vagina. If semen gets on the vulva or near the vaginal opening, it is possible for sperm to swim through moisture on the vulva into the vagina.
Some people have said this information is incorrect because the environment in the vagina is harsh and "toxic" to sperm. It's true that vaginas are acidic, but semen is alkaline (the opposite of acidic) and specifically designed to neutralize vaginal acidity and protect sperm. Cervical mucus, the fluid that comes from the cervix, is also alkaline and helps sperm survive and get into the uterus.
So, while it's very unlikely someone will get pregnant if semen only gets on the vulva or near the vaginal opening, it's not impossible — as several commenters have pointed out.
While this stuff may seem like common knowledge, we have a "there's no such thing as a dumb question" policy. Having the correct information about our bodies and sexual health is really important, and we believe in treating questions people ask with respect so everyone feels comfortable exploring this knowledge.
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Not far at all. In fact, get this: Semen (a.k.a. cum) doesn’t even have to be ejaculated into a vagina to cause pregnancy.
Semen — what comes out of the penis during ejaculation — contains millions of sperm cells, and it only takes one little sperm to cause a pregnancy. If semen gets ON the vulva or near the vaginal opening, the sperm cells that live in semen can swim into the vagina and fertilize an egg.
And, it’s not just semen doing all the work: The lubrication vaginas produce when sexy stuff is going on provides a slippery pathway into the vaginal opening. Imagine sperm have a very important appointment with an egg, and all those sexual fluids are their GPS.
So, while pregnancy is more likely to happen if sperm gets directly into the vagina, it can also happen if the vulva or vaginal area gets semen on it. This is why it’s so important to use condoms and birth control during sexual activity.
-Kendall at Planned Parenthood
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