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View Full Version : What's the chance of pregnancy for a single act of sex?


Blalron
06-19-2006, 06:43 PM
I hear that a fertile couple has a 90% chance of getting pregnant within a year if they aren't using protection, but what I haven't been able to gather is how often the average couple has sex.

What are the chances that any given instance of sex will result in pregnancy, assuming it's done at a random time and we don't know when the woman if ovulating?

SnakesCatLady
06-19-2006, 07:39 PM
I don't know the answer to the question, but I do know the U.S. Army considered a month's supply of condoms to be 9. This was in 1989 when I was on medication that required us to use condoms. My doctor laughed, told me what a month's supply was, and gave me a prescription for a 3 month supply.

racer72
06-19-2006, 07:47 PM
I can speak from experience, it takes only once.

ombre3
06-19-2006, 08:05 PM
I can speak from experience, it takes only once.

Very true.

It only takes one ticket to win the lottery too if you are very lucky.

I am also interested in the odds though of one single instance of a sexual encounter resulting in pregnancy.

How unlucky (or how lucky, depending on what you want) would you be-----stated simply in the odds for or against that happening?

(assuming normal fertility)

John Mace
06-19-2006, 08:17 PM
Very true.

It only takes one ticket to win the lottery too if you are very lucky.

I am also interested in the odds though of one single instance of a sexual encounter resulting in pregnancy.

How unlucky (or how lucky, depending on what you want) would you be-----stated simply in the odds for or against that happening?

(assuming normal fertility)
You can't calcluate those odds without makins some assumptions. If you assume a woman is fertile for "x" days per cycle, then the odds are x/c where "c" = days between cycles. The problem, though, is that you don't know the guy's sperm count or any number of other things that could affect a pregnancy happening.

Queen Tonya
06-19-2006, 09:53 PM
But aren't women more likely to conceive when they're younger? So age of the woman has to factor into the equation as well.

Yeah, my google-fu is weak but here's a weak cite (http://childbirthsolutions.com/articles/preconception/understanding/index.php)
The chances of becoming pregnant in any one month decrease from 20 percent in women over 30 years of age to five percent in women over 40.

Oooh, here's something else (http://healthguidance.org/articles/2849/1/Odds-Of-Getting-Pregnant-And-Age) : 1. For ages early to mid-thirties - a woman in general will be about 15-20% less fertile.
2. For ages mid to late thirties - fertility will generally decline by up to 50%.
3. For women ages early to mid-forties - fertility declines by over 90%.

Why do the odds of getting pregnant generally decline as the woman gets older? Studies have indicated the reasons for fertility decline are related to the quality of the woman's eggs as well as the quantity. A woman in her lifetime will typically produce about 400 fully developed eggs (usually one per month) that are capable of becoming implanted in the uterus and causing pregnancy. As these eggs get used up over thirty years or so and estrogen production slows so that the uterine and vaginal linings are no longer properly stimulated, pregnancy becomes less and less likely.

Blalron
06-19-2006, 09:56 PM
I'll simplify the question then.

The couple are the perfect representation of the Average Couple in terms of how often they have sex.

The man and women are the median age in the population of fertile persons.

John Mace
06-19-2006, 10:16 PM
The couple are the perfect representation of the Average Couple in terms of how often they have sex.
Married couple? Once or twice a year, is that it? :)

Snarky_Kong
06-19-2006, 10:31 PM
I'll simplify the question then.

The couple are the perfect representation of the Average Couple in terms of how often they have sex.

The man and women are the median age in the population of fertile persons.

Why will it matter how often they have sex? If I have sex once in my life or if I have sex every day, the chance that one single time is during a period when the woman if likely to get pregnant is the same. It's still the percentage of time she's fertile.

Queen Tonya
06-19-2006, 10:53 PM
I've seen 25% mentioned often on fertility sites, presumably that average is going with all the medians and averages possible. I dunno if things like conceptioncentral, fertilitext or babymed are providing viable information, or stacking the deck to make more people likely to seek fertility treatments.

Snarky_Kong
06-19-2006, 11:13 PM
I hear that a fertile couple has a 90% chance of getting pregnant within a year if they aren't using protection, but what I haven't been able to gather is how often the average couple has sex.

What are the chances that any given instance of sex will result in pregnancy, assuming it's done at a random time and we don't know when the woman if ovulating?

Using this number and the 9/month average I come up with... 2.1% of her getting pregnant every time.

X^(9*12)=100%-90%

where X is the chance of not getting pregnant each time, 9*12 is amount of sex in a year, and 100%-90% is the chance of not being pregnant at the end.

diggleblop
06-19-2006, 11:24 PM
99% chance of pregnancy from a single sex act if all the chemistry lines up.

Mayo Speaks!
06-20-2006, 02:23 AM
Using this number and the 9/month average I come up with... 2.1% of her getting pregnant every time.

X^(9*12)=100%-90%

where X is the chance of not getting pregnant each time, 9*12 is amount of sex in a year, and 100%-90% is the chance of not being pregnant at the end.
I agree with your calculation, but you need a big disclaimer that says:
THIS ASSUMES THAT THE NUMBERS IN THE FIRST TWO POSTS ARE CORRECT.

And I doubt that they are correct, although I can't provide cites.

Malacandra
06-20-2006, 04:32 AM
I don't know the answer to the question, but I do know the U.S. Army considered a month's supply of condoms to be 9. This was in 1989 when I was on medication that required us to use condoms. My doctor laughed, told me what a month's supply was, and gave me a prescription for a 3 month supply.

Wow, nine times a month? Do people really do it that often? :eek:

The horniest woman I ever dated did her damnedest to keep it down to twice a week, three weeks out of four. Oh the irony.

Nava
06-20-2006, 04:45 AM
Depends.

It's higher if the male has gone without for a while, including without visits to Mrs. Palm (more swimmers in the pool).

It's also higher if the girl has been saying "no" - Nature is called a Mother because she likes babies; many women get excited more easily when their fertility is higher. We're not that far from the other animals... (No cite, but I know several cases of cherry pregnancies)

Malacandra
06-20-2006, 07:49 AM
Depends.

It's higher if the male has gone without for a while, including without visits to Mrs. Palm (more swimmers in the pool).

It's also higher if the girl has been saying "no" - Nature is called a Mother because she likes babies; many women get excited more easily when their fertility is higher. We're not that far from the other animals... (No cite, but I know several cases of cherry pregnancies)

For sure. My parents married on 17th March 1951. Eldest sister arrived on 31st December. Unquestionably a legitimate conception but they didn't hang around.

Shagnasty
06-20-2006, 08:03 AM
I think the OP understands all the "It depends" and "the rate varies because" scenarios. You don't have to do an analysis with every possible permutation. Just throw all the fertile people into a big pile (no pun intended) and say that the rate of pregnancy is X for Y instances of intercourse for all fertile people.

Bear_Nenno
06-20-2006, 11:04 AM
I don't know the answer to the question, but I do know the U.S. Army considered a month's supply of condoms to be 9. This was in 1989 when I was on medication that required us to use condoms. My doctor laughed, told me what a month's supply was, and gave me a prescription for a 3 month supply.
You needed a PRESCRIPTION?? For condoms??

Ludovic
06-20-2006, 11:10 AM
You needed a PRESCRIPTION?? For condoms??Probably so they would be covered(heh) by insurance?

CynicalGabe
06-20-2006, 11:19 AM
The variables we need to know are:

1. Is the woman wearing a prom dress?
2. Does the act take place in the back of the car belonging to the parents of one of the parties involved?

If one or both of these are yes, then the chances are nearly 100%

Si Amigo
06-20-2006, 12:04 PM
I'd say 50-50. Either you get her pregnent or you don't.

groman
06-20-2006, 12:32 PM
I'm still amazed that there's only one even remotely plausible answer in this thread and it, unfortunately, has major problems with the methodologies.

What I remember from college bio is that the egg is only viable for about 48 hours a month. The sperm are a little better at about 72 hours. Not knowing the standard deviations of those numbers (I'm assuming they are means/medians) it's hard to give a probability breakdown but you can roughly estimate that you have about 120 hours on average every cycle when intercourse can result in pregnancy. Considering the average cycle length of 28 days (672 hours) the probability is about 18% of any given act of intercourse falling within the fertile period.

However, there's a few caveats. First and foremost having intercourse during the fertile window does not guarantee pregnancy and the numbers (which I can't find at the moment) would shock you. Second, out of all the pregnancies that would occur a large fraction will end in a miscarriage before they are detected. Thirdly when a woman is menstruating the chance of her becoming pregnant are slim to none (unless there's something non-standard going on pretty much none) - so this percentage is only meaningful during normal days, however when we calculated it we took the entire cycle length as our divisor.

From here (http://academic.sun.ac.za/medphys/Life01.html)

Fertilisation occurs in one of the uterine tubes usually within about 24 hours of ovulation. Sperms usually remain viable in the female genital tract for about 48 hours, but can stay viable for a week. The ovum also usually remains fertilisable for about 48 hours after ovulation. Artificial insemination studies suggest that if coitus takes place at the most ideal time for fertilisation (viz. during the 24 hours just before ovulation), then the chances of a viable pregnancy are, at best, only about 30%

So even if you take the very generous 30% and apply it to our entire window then at a random time during the menstrual cycle the probability of a single act of intercourse resulting in pregnancy is about 5% and that is a very very safe number. If you restrict intercourse to days when a woman is not menstruating then that number jumps to about 7%. The actual probability is probably significantly lower, especially for adults, I don't have good data to support this but my gut feeling tells me it should be under 1%.

I mean, this is not medical advice, this is just some haphazard handwavy math but this is the methodology you would use to determine the number you want by plugging in better input data.

Shagnasty
06-20-2006, 12:53 PM
I'd say 50-50. Either you get her pregnent or you don't.

That type of statement always made me furious for some reason (profound ignorance of statistics is one of the contenders). I am not saying you were serious but I have heard people use that logic seriously many times. No, your chance of winning the lottery is not 50-50 because you either win or you don't and likewise for all outcomes of that nature.

Pedro
06-20-2006, 01:07 PM
Assuming the woman is fertile for 14 out of 28 days in her cycle, and adding 3 days for the average expected life of sperm, gives 17 corrected fertility days.

So the probability of a woman being fertile is y = 17/28 = 0,607.

The probability of conception is more dicey because it will vary over the cycle. If the pregnancy is unwanted let's assume a worst case of x = 0.36, the probability of conception assuming the woman is fertile (taken from here (http://clearplan.com/canada/Wilcoxnewspage.cfm)).

So P(x and y) = 0,607 * 0.36 = 0.22

So by my crude estimate you would have approximately a one in five chance of pregnacy in a single roll of the dice.

groman
06-20-2006, 01:09 PM
Assuming the woman is fertile for 14 out of 28 days in her cycle, and adding 3 days for the average expected life of sperm, gives 17 corrected fertility days.

Where did you get the 14 out of 28 days number? The egg is only viable for about 48 hours average. The 14 days number is the window during which ovulation can occur, which is not meaningful in such a calculation. Within that 14 day window there's a 48 hour window where conception can ACTUALLY occur.

Pedro
06-20-2006, 01:55 PM
Where did you get the 14 out of 28 days number? The egg is only viable for about 48 hours average. The 14 days number is the window during which ovulation can occur, which is not meaningful in such a calculation. Within that 14 day window there's a 48 hour window where conception can ACTUALLY occur.

You're right, let me correct the math:

P(woman is fertile) = 5 days / 28 days = 0.179 (three days before to two days after ovulation).

P(conception given that woman is fertile) = 0.36

P(pregnancy) = 0.36 * 0.179 = 6.4%

Seems kind of low, this is assuming a highest case of 0.36. Maybe I messed up again.

guizot
06-20-2006, 02:14 PM
I had an English professor once say in lecture, that, regarding the Scarlet Letter, Esther (or whatever her name was) had very bad luck. He said pregnancy is 1 in 8 random times.

groman
06-20-2006, 02:18 PM
You're right, let me correct the math:

P(woman is fertile) = 5 days / 28 days = 0.179 (three days before to two days after ovulation).

P(conception given that woman is fertile) = 0.36

P(pregnancy) = 0.36 * 0.179 = 6.4%

Seems kind of low, this is assuming a highest case of 0.36. Maybe I messed up again.

See my calculations above. Why does that seem low? My intuition tells me less than 1% for adults, maybe around 1% for teenagers. I mean, I'm applying common sense to our calculations: Say your figure is indeed low, and the actual number is 10%. That means that if you get 100 couples to have sex once, 10% of the women would get pregnant, that doesn't sound even remotely plausible.

A good rule of thumb is that what the average joe thinks is reasonable is usually at least an order of magnitude off. For example, I recently read a statistic that says the probability of a female-to-male tranmission of HIV from one act of unprotected intercourse with an HIV positive partner is 0.2%. That's right, 0.2%. The probability, IIRC, with a properly used condom was only 8 times better. Any random person I ask about this thinks without a condom it's 99% and with a condom 0.1%. Obviousness can be very deceiving.

Si Amigo
06-20-2006, 03:30 PM
That type of statement always made me furious for some reason (profound ignorance of statistics is one of the contenders). I am not saying you were serious but I have heard people use that logic seriously many times. No, your chance of winning the lottery is not 50-50 because you either win or you don't and likewise for all outcomes of that nature.

It was meant to be flippet. But that's what goes thru a teenage boys mind when he thinks his girlfriend might be knocked up. I remember when they didn't have home pregnancy test that you could just get down at drug store and have results that very day. Alot of the wildass guessing and calculating goes thru a persons mind, just as this thread has shown. Not logical at all. And yes, downright ignorant. :dubious:

mr. jp
06-20-2006, 04:37 PM
A good rule of thumb is that what the average joe thinks is reasonable is usually at least an order of magnitude off. For example, I recently read a statistic that says the probability of a female-to-male tranmission of HIV from one act of unprotected intercourse with an HIV positive partner is 0.2%. That's right, 0.2%. The probability, IIRC, with a properly used condom was only 8 times better. Any random person I ask about this thinks without a condom it's 99% and with a condom 0.1%. Obviousness can be very deceiving.
I believe you, but can I please get a cite? This fact would be important to me.

groman
06-20-2006, 04:43 PM
I believe you, but can I please get a cite? This fact would be important to me.

I don't have the study I was referring to handy but here's a slightly different study with even more surprising numbers: http://cirp.org/library/disease/HIV/gray2/

Pedro
06-20-2006, 06:18 PM
See my calculations above. Why does that seem low? My intuition tells me less than 1% for adults, maybe around 1% for teenagers. I mean, I'm applying common sense to our calculations: Say your figure is indeed low, and the actual number is 10%. That means that if you get 100 couples to have sex once, 10% of the women would get pregnant, that doesn't sound even remotely plausible.

A good rule of thumb is that what the average joe thinks is reasonable is usually at least an order of magnitude off. For example, I recently read a statistic that says the probability of a female-to-male tranmission of HIV from one act of unprotected intercourse with an HIV positive partner is 0.2%. That's right, 0.2%. The probability, IIRC, with a properly used condom was only 8 times better. Any random person I ask about this thinks without a condom it's 99% and with a condom 0.1%. Obviousness can be very deceiving.

Well said. My intuition was geared towards intercourse over a longer period of time. Sorry for missing your previous post.

Velma
06-20-2006, 09:44 PM
groman's figures are correct according to most fertility sites I have read. When I was trying to get pregnant I read everything I could, and charted my fertile days each month. For the average cycle of 28 days, the woman ovulates around day 14 and the egg is available for 24-48 hours. The sperm can live in the woman for 3-4 days max. So his calculations seem about right.

The commonly cited "1 in 4" odds are what I always read as the odds that a woman will get pregnant while having sex during her fertile time in a given month. So supposedly you have a 25% chance of getting pregnant each month you "try." However, you also have to consider that egg+sperm fertilization does not always = pregnancy. So if you are asking what are the odds that a one time sex act will result in a viable pregnancy that results in a live birth, that is a little different than asking what are the odds an egg will be fertilized.

Rick
06-21-2006, 12:05 AM
All I have to add is that in my experience the odds are 100%. One time is all it took.
The best daughter in the world just turned 21. :D

Pedro
06-21-2006, 08:16 AM
A point worth noting is that a one night stand is unlikely to happen during menstruation (also, that is an obvious condition that significantly lowers the probability of pregnancy). With that caveat, if we rule out 5 days of the cycle because of menstruation, the probability of pregnancy rises to aproximately 7.9% according to my numbers.

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