I’d say 50-50. Either you get her pregnent or you don’t.
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
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.
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.
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).
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.
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.
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.
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.
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:
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://www.cirp.org/library/disease/HIV/gray2/
Well said. My intuition was geared towards intercourse over a longer period of time. Sorry for missing your previous post.
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.
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. 
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.