What's the chance of pregnancy for a single act of sex?

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?

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.

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)

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.

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

Oooh, here’s something else :

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.

Married couple? Once or twice a year, is that it? :slight_smile:

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.

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.

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


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.

99% chance of pregnancy from a single sex act if all the chemistry lines up.

I agree with your calculation, but you need a big disclaimer that says:

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

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.


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.

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.

You needed a PRESCRIPTION?? For condoms??

Probably so they would be covered(heh) by insurance?

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%