# What's the chance of pregnancy for a single act of sex? **URL:** \ **Category:** Factual Questions\ **Created:** [June 19, 2006, 11:43pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363 "2006-06-19T23:43:54Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Blalron](https://avatars.discourse-cdn.com/v4/letter/b/35a633/32.png) [@Blalron](https://boards.straightdope.com/u/Blalron)\ **Post date:** [June 19, 2006, 11:43pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/1 "2006-06-19T23:43:54Z")
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? ---
**Author:** ![SnakesCatLady](https://avatars.discourse-cdn.com/v4/letter/s/4af34b/32.png) [@SnakesCatLady](https://boards.straightdope.com/u/SnakesCatLady)\ **Post date:** [June 20, 2006, 12:39am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/2 "2006-06-20T00:39:15Z")
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. ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [June 20, 2006, 12:47am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/3 "2006-06-20T00:47:03Z")
I can speak from experience, it takes only once. ---
**Author:** ![ombre3](https://avatars.discourse-cdn.com/v4/letter/o/da6949/32.png) [@ombre3](https://boards.straightdope.com/u/ombre3)\ **Post date:** [June 20, 2006, 1:05am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/4 "2006-06-20T01:05:06Z")
> [@racer72](#): > > 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) ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [June 20, 2006, 1:17am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/5 "2006-06-20T01:17:39Z")
> [@ombre3](#): > > 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. ---
**Author:** ![Queen\_Tonya](https://avatars.discourse-cdn.com/v4/letter/q/4491bb/32.png) [@Queen\_Tonya](https://boards.straightdope.com/u/Queen_Tonya)\ **Post date:** [June 20, 2006, 2:53am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/6 "2006-06-20T02:53:15Z")
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://www.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://www.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. ---
**Author:** ![Blalron](https://avatars.discourse-cdn.com/v4/letter/b/35a633/32.png) [@Blalron](https://boards.straightdope.com/u/Blalron)\ **Post date:** [June 20, 2006, 2:56am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/7 "2006-06-20T02:56:29Z")
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. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [June 20, 2006, 3:16am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/8 "2006-06-20T03:16:33Z")
> [@Blalron](#): > > 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? 🙂 ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [June 20, 2006, 3:31am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/9 "2006-06-20T03:31:15Z")
> [@Blalron](#): > > 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. ---
**Author:** ![Queen\_Tonya](https://avatars.discourse-cdn.com/v4/letter/q/4491bb/32.png) [@Queen\_Tonya](https://boards.straightdope.com/u/Queen_Tonya)\ **Post date:** [June 20, 2006, 3:53am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/10 "2006-06-20T03:53:55Z")
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. ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [June 20, 2006, 4:13am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/11 "2006-06-20T04:13:38Z")
> [@Blalron](#): > > 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. ---
**Author:** ![diggleblop](https://avatars.discourse-cdn.com/v4/letter/d/ac91a4/32.png) [@diggleblop](https://boards.straightdope.com/u/diggleblop)\ **Post date:** [June 20, 2006, 4:24am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/12 "2006-06-20T04:24:46Z")
99% chance of pregnancy from a single sex act if all the chemistry lines up. ---
**Author:** ![Mayo\_Speaks](https://avatars.discourse-cdn.com/v4/letter/m/c57346/32.png) [@Mayo\_Speaks](https://boards.straightdope.com/u/Mayo_Speaks)\ **Post date:** [June 20, 2006, 7:23am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/13 "2006-06-20T07:23:08Z")
> [@Snarky\_Kong](#): > > 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. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [June 20, 2006, 9:32am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/14 "2006-06-20T09:32:49Z")
> [@SnakesCatLady](#): > > 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. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [June 20, 2006, 9:45am UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/15 "2006-06-20T09:45:49Z")
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) ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [June 20, 2006, 12:49pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/16 "2006-06-20T12:49:22Z")
> [@Nava](#): > > 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. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [June 20, 2006, 1:03pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/17 "2006-06-20T13:03:09Z")
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. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [June 20, 2006, 4:04pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/18 "2006-06-20T16:04:42Z")
> [@SnakesCatLady](#): > > 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?? ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [June 20, 2006, 4:10pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/19 "2006-06-20T16:10:01Z")
> [@Bear\_Nenno](#): > > You needed a PRESCRIPTION?? For condoms?? Probably so they would be covered(heh) by insurance? ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [June 20, 2006, 4:19pm UTC](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363/20 "2006-06-20T16:19:15Z")
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% [Next page](https://boards.straightdope.com/t/whats-the-chance-of-pregnancy-for-a-single-act-of-sex/361363.md?page=2)