How often does pregnancy result from a single encounter?
In old movies a boy and girl went out once and made love and he went off to war or something and she was pregnant.
In the abortion debates exceptions are often argued from pregnancy resulting from rape.
Someone, somewhere must have done the statistics. Do you know how often such single incidents result in pregnancy?
NM
Let’s see… assume her cycle is 28 days long. Of those days, she’ll be fertile about 4 of them. That means a random PIV sexual encounter has about a 1/7 chance of hitting during a fertile day.
There are other factors, but that’s a place to start.
um…every time? I’ve not known of pregnancy requiring multiple encounters.
Assuming young fertile, virile, healthy individuals, I’d assume the 1/7 number is a pretty good starting point. That doesn’t mean the encounters were randomly selected like that, but the odds of impregenation have to be pretty good. Otherwise there’d be a lot less people in the world, and fewer attempts to proscribe sex.
Since the starting point of reasoning this question is “old movies,” I’d say it happens 100% of the time the plot requires it.
I don’t know. Wouldn’t it be hard to find women who hadn’t used any birth control on their first time and waited longed enough before their second to really know it was the first that did it?
I see from one site that the pregnancy rate given no birth control method at all is 85%. Those sites never define what they mean by pregnancy rate, but I believe it’s a measure of how many women get pregnant in a year of using that method. Now, let me further WAG that a year of sex means 100 times. If you’re doing better than that, congratulations. Work out your own damn formula. I’ll also assume that your chance of getting pregnant the first time is equal to the chance of any other given time, though I have no idea if that’s true.
So, by the binomial probability equation:
Probability of 0 pregnancies = (100 choose 0) x p^0 x q^100 = 0.15
p is the probability of getting pregnant on a given trial
q is the probability of not getting pregnant that trial (i.e. 1 - p)
Solving for p gives just under a 2% chance of getting pregnant on your first try … if my assumptions and math are correct.
Just in case there’s a reason behind the question, 2% is still an unacceptably high risk even if it’s accurate. Use protection.
That’s not exactly right. The egg is viable for 24 to 48 hours (not four days). Sperm can survive for up to five days. Therefore, there is a 7 day window for pregnancy each month, assuming healthy, young, fertile individuals.
For additional information, there is about a 25% chance of a woman without fertility issues getting pregnant in a month if she has sex during those fertile days.
So, and I may not be remembering my statistics right 7/28 = .25 * .25% = 6.25%
There’s also a non-negligible chance of a frazzled, embarrassed and terrified young woman blurting out “but we only did it once” when faced with the news. It’s a weird, illogical attempt to minimize responsibility–it’s bad enough that your mom has to find out you had sex, you’d rather her think it was a one time mistake. Once you’ve made that claim, you pretty much have to stick with it.
Twice for me.
During ovulation a woman is also more receptive to sex, and can be a little more careless about protection. “But we ALWAYS used protection! Except that ONE time when we got carried away…”
This will vary greatly with the woman’s age, of course.
To reiterate what Manda JO said:
According to Gregory House, MD, “Everybody lies.”
~VOW
As a married man with 3 kids…
Our first two… we were every other day besides one for the cycle using protection… but we decided… “oh what the heck, it would be fun if”.
So meet our first two children.
The third took 3 months of active trying though. But yeah especially in those younger years… it’s extremely easy.
This is what I understand as well. Why do they use this method? Doesn’t it vary widely with how often the woman has sex? That’s like rating car mileage by how many times a year you top up the fuel.
Contraception effectiveness rates are determined by asking women (usually at OB/Gyn appointments) what form of contraception they used in the last year and if they got pregnant on it. If 100 women report using Phlebotinum as contraception for 1 year and 20 of them get pregnant, the effectiveness of Phlebotinum is listed as 80%.
It doesn’t really take a statistician to see the problems with this system, but no one’s come up with a better alternative…this isn’t exactly an area where double blinded placebo controlled studies are ethical.
I am not sure it matters as much as it might seem at first. As long as a woman is having sex 2-3 times a month–and I think most people using any form of birth control are probably usually having sex at least twice a month–then some of that sex is going to be during the fertile period for some of those months. Having more sex during the fertile period doesn’t have that dramatic of an effect: once you get over a certain threshold of sperm, more are superfluous.
Now, if you just looked at stats for a month, obviously the woman having sex every other day would have been more likely to have sex during her potentially fertile period. But over the course of 12 months the less-frequent woman would “test the system” several times. The more-frequent sex-haver might get pregnant earlier, but it would average out over a year.
But if sperm can live upto 4 days doesn’t that increase the window of opportunity to possibly 8 days? That makes it more like 1 in 4.
(but wth do I know, I have had a vasectomy for as long as I can remember)
Yep, the 25% is an approximate average. It is likely to be higher for a younger woman and lower for an older woman. It will be higher if fraternal twins run in your family (you probably release more eggs monthly than other people).
On the 7 days of fertility, the cycle is 28 days long, but some people don’t like sex during their (or their partners) periods. In which case your chances go up. And I’m guessing that if what you are talking about is a girl losing her virginity, her chances of doing that during her period are much lower than choosing the smack dab in the center of her cycle - when she is most fertile.