What percentage of unprotected intercourse results in pregnancy?

February 9, 2005 8:39 PM

What percentage of unprotected intercourse results in pregnancy?

More specifically: If a randomly chosen male from the set of all sexually active males and a randomly chosen female from the set of all sexually active females have unprotected vaginal sex exactly once, what is the likelyhood of a resulting pregnancy?

posted by event to health & fitness (29 comments total)

It seems to be a bit Engrish, but Pregnancy Odds has a good overview of the problem. Short answer: roughly 2.5%.
posted by dhartung at 8:56 PM on February 9, 2005

If they're both completely fertile, and ovulation has taken place (generally 14 hours after the LH surge), and it's within a day of the ovulation (eggs only last 24-48 hours), and there are no other complicating factors, and she's not on any oral contraceptives, and she doesn't have any conditions that might affect her ability to get pregnant (past history of PID, etc), I've heard 80%.

But if your question involves people's odds of infertility, etc, that's an entirely other story, and probably unanswerable. Infertility of men and women is fairly common; many women of reproductive age are on the pill; the older men get, the worse their sperm are; women of reproductive age could have had hysterectomies... none of that even takes into consideration that 26 out of 28 days of every cycle, women aren't ovulating... it's amazing any of us are here at all.

That being said, the number of unexpected pregnancies is generally quite high.
posted by gramcracker at 9:07 PM on February 9, 2005

In Sex Ed class, to me, this is the first and most obvious question to ask, followed by, "What are the chances of catching an STD from a random sex act?"

I recall being told all kinds of horrible numbers in answer to the latter question, but nothing regarding the former. My suspicion as to the reason for this, which dhartung seems to have confirmed, was that the percentage would be quite low.
posted by event at 9:29 PM on February 9, 2005

A fertile couple can pop out a baby a year without much effort at all. And, indeed, most couples do seem to get their average 2.15 children per household without any great difficulties: when they decide to pop out a sprog, they have sex a few times a week for a month or three. If the rate were truly so low as 2.5%, most couples would have to screw for fifty years before the odds were in their favour.

Or I'm completely misunderstanding what "a 2.5% chance of getting pregnant" actually means.
posted by five fresh fish at 10:21 PM on February 9, 2005

2.5% per act of sex.

If you're only getting laid once a year, it'd take quite a while, yes. But 2.5% per time means that after trying 100 times (married couples - 1 year, new lovers - 1 month), you're probably gonna get knocked up.
posted by u.n. owen at 10:35 PM on February 9, 2005

You might as well ask "what are the odds that a given woman is ovulating at any random moment?" That's the biggest factor in the final number you're looking for.
posted by scarabic at 10:59 PM on February 9, 2005

Researching this (for no particular reason) it actually became a pretty interesting question. I realized that when I was looking up contraceptive effectiveness statistics, generally nothing was said about methodology. I'm guessing that the majority of statistics are rooted in statements like this (from http://lib-sh.lsuhsc.edu/fammed/grounds/cntrcpt.html) -

"With no contraception, it is estimated that 85% of average couples who engage in regular intercourse will conceive within one year. The effectiveness of different contraceptive methods is usually described in terms of "perfect use" (annual rates of pregnancy among couples who use the method correctly at every act of intercourse) and "typical use" (pregnancy rates among average users in retrospective trials or clinical studies)."

I'd question the analysis provided by dhartung because a lot of fudge factors are used and there are no data sources ("Consider with a regular 28 day period," "You will be fertile, practically, for two days" - emphasis added - the "broad" fertile period is more like six days, the author acknowledges that sperm viability varies, and no source for the statistic is given that pregnancy will occur 25% of the time when sperm and egg are together). The complications are endless. Female orgasm increases the chance of conception, for example - try to calculate for that one. I think what that author reports is 2.75, not 2.5 incidentally (11 divided by 4, yes?)

I don't think a real answer is possible.
posted by nanojath at 11:20 PM on February 9, 2005

Keep in mind that there has been a longstanding belief that all women ovulate once every menstrual cycle approximately 14 days before their period. This has been proven false. This is a huge part of why the rhythm method doesn't work as well as it should and why unprotected sex results in more pregnancies than one might previously expect. See link here.
posted by hindmost at 11:37 PM on February 9, 2005

Let's not jump to conclusions, hindmost. That's one study, n=63. There's much more research needed to confirm that initial finding. The pre-ovulatory cycle can vary a bit, but according to my ob-gyn professors, the post-ovulatory cycle is, in most women, 14 days. There's lots of reasons the rhythm method is fairly ineffective; that study highlites one.
posted by gramcracker at 12:10 AM on February 10, 2005

scarabic: Well, it's the percent of time that they're fertile, times the percent chance of concieving during that time. Two different things, granted, but the other one is probably as important to the overall figure. Splitting hairs, though.
posted by abcde at 12:30 AM on February 10, 2005

If both the male and female are fertile, pretty high. Women are more interested in having sex when they're ovulating, therefore random sex is going to have a higher rate of pregnancy than you'd think otherwise. Other conditions may apply, including the health of both partners and recent sexual activity.

In another light, the chances are high enough that it'd be entirely unreasonable to have completely unprotected sex if there's a desire to not cause pregnancy. 2.5 percent is still a higher chance than you hitting any given number on a roulette wheel.

I don't have the statistics for it on hand, but I'm sure you could find more reliable information on average amounts of sex a married couple has by age, and average time required to conceive, by age. It'd be pretty easy to get a rough number from that.
posted by Saydur at 12:50 AM on February 10, 2005

2.5 percent is still a higher chance than you hitting any given number on a roulette wheel.

No it isn't, chances of hitting a given number on a roulette table are 2.63% (American table, 38 numbers, 0 and 00) or 2.7% (European table, 37 numbers).
posted by biffa at 2:54 AM on February 10, 2005

I realize that this is a pointless nitpick, but studies suggest that a large number of "pregnancies" spontaneously abort before or around the 4 week mark, before the woman has reason to believe she's pregnant. Many of these zygotes have non-viable chromosomal aberrations in the few studies that have been done, and this is probably where nanojath's 25% comes from. Whether you count this as a pregnancy or not is sort of definitional, but when it takes place in a typical one-ovum ovulatory cycle, no other pregnancy can occur during that cycle.

Gambling with the production of a human life seems extraordinarily irresponsible to me. I'd leave odds out of any reproductive health class I taught, for this reason and for the reason that people vary so widely from the mean in this respect.
posted by ikkyu2 at 4:46 AM on February 10, 2005

I was just going to mention what Saydur said, women are far more interested in having sex right before and during ovulation, I can vouch for that.
posted by dabitch at 5:45 AM on February 10, 2005

Several of my wife's med-school friends are trying to conceive right now (well maybe not right now). They've mentioned numbers around nanojath's 25% for conception rates when you're following the rhythm method. They didn't cite a study, though.
posted by Plutor at 6:42 AM on February 10, 2005

From the FAQ at Not-2-Late.com:

What is my risk of pregnancy from unprotected intercourse?


The risk of pregnancy is virtually 0 during the first two days of the cycle (day 1 of the cycle is the first day of bleeding). The risk begins to rise steadily thereafter, reaching 9% on about day 13, begins to decline slowly thereafter to about 1% on day 25, and remains at about 1% as late as day 40 and beyond. (Average cycle length is 29 days, but it is normal to have cycles that last anywhere from 20 days or less to 40 days or more.) However, it is important to note that these figures are averages and that the risk for an individual woman may be higher or lower.
posted by v-tach at 7:09 AM on February 10, 2005

And if you find that you've had unprotected sex (in the case of rape or drunkeness or whatever) remember that if pregnancy is not desired, there is an emergency contraceptive pill that can be taken anytime in the 72 hours after intercourse. Its efficacy decreases as time passes, so the sooner is better. In Canada, this product is available from your pharmacist without a perscription (but it should not be relied upon regularly, as repeated use reduces its effect).

Point being? You don't have to rely on "the odds" even if you made a mistake in the heat of the moment.
posted by raedyn at 7:20 AM on February 10, 2005

Given that the Pill is considered highly effective and still has a, what, 1% failure rate?, I really doubt that unprotected sex is only twice or thrice more likely to produce babies.
posted by five fresh fish at 10:21 AM on February 10, 2005

five fresh fish, that's not per time, that's per year.

To recap: unprotected sex, 2.5% PER TIME.

Pill, 1% per year.
posted by u.n. owen at 10:55 AM on February 10, 2005

Pill, 1% per year.
Aahh! I have long wondered about this actually - I always thought it was weird that a 1% failure rate would be considered good enough when it came to pregnancy, considering you would only have to have sex 100 times to conceive, and therefore pretty much any couple would be pregnant in a year. so thanks for clearing that up.
posted by mdn at 11:17 AM on February 10, 2005

All contraceptive methods are measured on a per-year basis. It goes a little like this (FIGURES ARE NOT EXACT):

Surgical methods: .01-.1% failure (depends on method)
Depo Provera: .5% failure
IUDs/Pill/ring/hormone patch: 1% failure
Condoms, perfect use: 2% failure
Condoms, normal use: 7-10% failure
Rythm method: 34% failure
No method: 85% failure.

Those are over the course of one year. They obviously are not true per time. I'm glad we could clear all this up, it's one of the leading misconceptions in abstinence only sex ed. Kids hear "Condoms fail up to 10% of the time? She'll get knocked up after 10 times anyway, so why bother?"
posted by u.n. owen at 12:00 PM on February 10, 2005

Okay - for the purposes of that argument, how many sex acts are there in a year?
posted by coriolisdave at 3:09 PM on February 10, 2005

Okay - for the purposes of that argument, how many sex acts are there in a year?

Yeah... and why don't they just divide them by this to get a number that will be the same for everyone? ie, if 100x a year is considered an average (erring on the low side), then the pill would fail .01% of the time, or 1 in 10000 times. This would sound much more effective than the way we currently explain it. Why don't birth control advocates clarify this?
posted by mdn at 3:34 PM on February 10, 2005

"At any given moment, 2,000 couples around the world are having sex."

"On average, people across the world are having sex 97 times a year."

"The Americans have sex more than any other nation at an average of 124 times a year, followed by the Greeks (117), South Africans and Croatians (both 116) and New Zealanders (115)."

Just the results of a bit of googling. To really blow your mind, assume that only people between 15 and 65 have sex planetwide, assume half are guys, that's c. 1 183 298 295 guys. So picking randomly, let's say that 70% of those guys have sex per year, and they shoot nice healthy sperm every time. Yes, I am finding this funny! :)

That's 1 449 540 410.5 teaspoons of sperm flying per year. 1 887 422.41 gallons. That could kill 21 people if it was hot molasses. *grin*
posted by paperpete at 4:17 PM on February 10, 2005

Imagine each sperm was a whale, travelling Mach 3 on scale, heading from the California to the coast of Japan.

- c.f. Laurie Anderson, Home of the Brave.
posted by five fresh fish at 6:59 PM on February 10, 2005

So with Pill, one in ten thousand.

Without Pill, three in one hundred.

If they were bullets, would you pull the trigger?
posted by five fresh fish at 7:01 PM on February 10, 2005

Change that.

With the pill, one in ten thousand.

Without the pill, one in twenty five.

I know that'd've alarmed me back in my horny teen days.
posted by five fresh fish at 7:04 PM on February 10, 2005

Without the pill, one in twenty five.

2.5% = one in forty
posted by biffa at 6:36 AM on February 11, 2005

d-oh!
posted by five fresh fish at 10:19 AM on February 11, 2005