General Question

Jack79's avatar

What is the statistical possibility of a girl getting pregnant?

Asked by Jack79 (10984points) December 1st, 2009

Before you guys freak out, no, I did not bang anybody up. This is purely a statistical question, and I need it for a D&D game I’m playing. One of the players today had sex with an NPC. She is a human sorceress (healthy 25 yo) and the NPC was a 50-y-old nobleman (also healthy). Regardless of the storyline, I wanted to know what her chances are of her getting pregnant, so as to include it in the game.

So, assuming neither person is sterile (or we could also add that into the equation), and notwithstanding the possibility that she may have had her period (something I need to take into account in the future it seems) what is the statistical chance of someone getting pregnant when having a one-night-stand?

for those unfamiliar with D&D rules, there is a combination of 2 dice that allows you to roll numbers between 1 and 100, so if I know the chance is 13% for example, I’ll have her roll the dice, and if the number is between 1 and 13 she’ll be pregnant, if higher, she won’t

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34 Answers

andrew's avatar

If I were DMing the campaign, I would totally fudge things to make her pregnant. It’s so much more interesting for the storyline, don’t you think? Even you don’t see the child for a while and you bring it back as a surprise in a year. It’d be epic.

Besides, she’s a sorceress. I think odds are out the window on this one. Isn’t fertility a cantrip?


andrew's avatar

Oh, sorry. I read it as the reverse.

Jack79's avatar

yeah as a DM I can do anything, including assuming she’s infertile (or just got pregnant on the first shot). But I was just wondering what the actual chances are in the real world. A healthy 25-y-old woman with a healthy 50-y-old man, doing it once.

holden's avatar

I’d guesstimate 25 percent.

casheroo's avatar

Well, I was always told you have a 20% chance.

And the age of the male doesn’t really matter, they can almost always reproduce (since we know there’s no fertility issue) A lot of factors go into it though when it comes to the woman. Like, when during their cycle, if she had irregular periods to begin with…etc.

Here’s a little break down.

* There is about a 15–25% chance of becoming pregnant in each ovulatory cycle: But you need to do everything right and you shouldn’t have any medical issues and his sperm count should be fine.
* Approximately 40% of couples who are starting out TTC will conceive during the first three months of trying, and about 70% of couples TTC will conceive during the first six months.
* Up to 85% of couples will conceive within the first year of TTC. Your ability to conceive in any given cycle falls after age 30.

And if she hasn’t been on any birth control, she might conceive sooner because he body doesn’t need to rid itself of the BC hormones.
I’d stick with 20%

gemiwing's avatar

Male fertility decreases with age too- so that would need to be added into the factoring.

tyrantxseries's avatar

did she cast a condom spell beforehand?

Jack79's avatar

No condoms in that particular universe (she did actually ask me about that, funnily enough). And the world ends within the year if they don’t save it on time, so she probably won’t have time to give birth anyway.

So, if I understand casheroo right, there’s a 20% of a girl getting pregnant if she has sex on a random day within her cycle? Remember we’re talking about a one-night-stand only, not a couple trying to reproduce.

So let’s say her cycle is 30 days, and 5 of them are around her period. Would she get pregnant on 5 of the remaining 25, and not get pregnant on 20/25? I’m just looking for the simplest statistical formula, rather than an understanding of the medical implications of fertility.

I always assumed the chances were less. 20% makes things quite interesting I guess :)

casheroo's avatar

And then you have to factor in that many, many pregnancies end in miscarriage…many before the woman even knows she’s pregnant.

Simone_De_Beauvoir's avatar

so glad you didn’t ‘bang anybody up’

grumpyfish's avatar

Roll for pregnancy… if pregnant, it would reduce your CON. After the 1st Trimester, would also reduce your INT, but increase your CHA. =)

After each battle you’d need to roll for a miscarriage.

galileogirl's avatar

Healthy early 20’s with a regular sex life w,i 18 mos, 95%. If you are really trying and using a calendar & thermometer and a fertility kit, 80% in 3 mos-95% in 6 mos

ratboy's avatar

Do some empirical research. Have unprotected sex with a thousand young women, and determine how many become pregnant. If you’re much younger than fifty, ask your father to do the research for you.

MagsRags's avatar

20–25% for a single cycle with good timing sounds about right. Other confounding factors:

The egg is only good for about 24 hours, but sperm are viable for 3–5 days after ejaculation, meaning if sex happens a couple days before ovulation, the sperm can by laying in wait for the egg to arrive into the fallopian tube.

Ovulatory pheromones might increase the potential for the couple to have succumbed to their mutual attraction at her fertile time.

Is she in the habit of having unprotected sex? If so and she hasn’t gotten pregnant yet, she might not be as fertile as the average woman.

Overall, I’d probably go with a 1 in 10 chance of pregnancy. As far as miscarriage goes, one in four ends in the first 3 months. If she’s taking blows to the abdomen, all bets are off.

RedPowerLady's avatar

I think @casheroo has a great answer.
Also @MagsRags

Zuma's avatar

Your odds of getting pregnant due to a single act of unprotected sex are 4%, or 1 chance in 25, on average.

Fertility, however, declines with age. For a woman under 30, her chances would be 5% or one in 20. For a woman ages 35–39, her chances are 2.5% or one chance in 40 (at the mid-point). For a woman over 40, her chances decline to .5% or one chance in 200.

However, 15% of pregnancies miscarry; so the odds of your character having a baby 9 months later from a single act of sex are roughly 1 in 24.

Jack79's avatar

@MagsRags see, this is what I always assumed when trying to either get/not get someone pregnant. So that means that there is one day out of 30 (even if you don’t exactly know which one it is) that would be perfect, then say +4 days for the sperm, so that’s 5/30=16%

Again, I am not trying to figure out how to make babies. I only want the statistical chance of someone getting pregnant if she has sex once (imagine she only has sex once in her life). Not knowing which day of her cycle that is (theoretically it could also fall within her period) and not trying to either stay or not stay pregnant. Just the statistical chance of a random one-night-stand within an average healthy woman’s cycle (and assume the cycle is 30 days).

Jack79's avatar

@Zuma perfect! thanks, that’s exactly the answer I was looking for. So 5% in this case, plus 15% chance of miscarriage (which as grumpyfish suggested, I will roll after every battle). The particular character doesn’t strain herself much, though she did have to spend a few days tied up in a dungeon. She generally just spends time in cities though, or casting spells from a distance, so she should be ok. Oh and she rolled a 98, so she didn’t get pregnant anyway, but I wanted the rule for next time she tries that trick on NPCs.

@magsrags we just added this character to the party (level 5). We assume she was not a virgin, but there was no reason to discuss her sexual life before she entered the story. She generally spent a lot of time alone, was not married, and was very beautiful and used her charm to seduce men (and can also cast a powerful “charm” spell). She was not planning to seduce this nobleman, but the story just turned out that way. I was looking for a rule I can use in these cases, and wanted it to be realistic. In movies whenever a couple have sex, there’s always a healthy smiling baby 9 months later, especially if the characters are Kings and Queens. I wanted to base the game rule on real statistics.

Thank you all for your answers :)

galileogirl's avatar

@laureth It certainly helps older women!

MagsRags's avatar

If I were a powerful sorceress, part of my “charm” spell would include control of my fertility. If she has healing powers, it would be an entry level skill.

Jack79's avatar

@MagsRags nah, I’m afraid D&D rules are slightly more complicated than that. “charm” works on other people (you make them like you) and then you have to persuade them to do things for you (rolling dice). Healers are a completely different class, and I’m not even sure there’s a spell that can allow them to control their fertility.

I’ll also have to look up the chances of catching a STD in that world. Though I doubt that girl will be having any more sex after that last encounter (oh, and one of the other characters stole her clothes hehe).

grumpyfish's avatar

@Jack79 Mrs. Grumpyfish pointed out to me that Fish Oil can counteract the INT hit—so perhaps someone (or herself) could make some Fish Oil…

PandoraBoxx's avatar


The likelihood of becoming pregnant from a single act of unprotected sex (for example, from a one night stand) varies from person to person, and also depends on the stage of a woman’s menstrual cycle. The probability is highest around the time of ovulation (when the egg is released), when, on average, up to one third of women will become pregnant from having sex once.

DrBill's avatar

Odds of becoming pregnant with only one sexual event is 1 in 8.

galileogirl's avatar

@DrBill I have to know where that came from!

DrBill's avatar

The average healthy fertile woman can become pregnant 3 days a month on average, of the 24 days she is not on her period. 3/24=8

galileogirl's avatar

Fortunately women are not strictjy mathematical.

Zuma's avatar

@DrBill Even if you have sex with a woman on one of her fertile days, it is not 100% sure that she will become pregnant. Rather, there is a 1 in 4 (or less) chance on one of those days. So, the overall oddsa are about 4% or one in 25 (see my post above).

Jack79's avatar

ok, so what I did is this: I included the period days into the cycle, since we’re not keeping track of the imaginary woman’s cycle as we’d do with a real person. Otherwise, I’d have to ask my friend whether she has her period in RL, but that wouldn’t work, because we only play on Tuesdays and Thursdays, and she’d probably just not show up to play if she had her period.

So I simplified the formula to assume that a woman can get pregnant on 5 days a month, which would make it also easier in terms of game rules: you just roll one d6, and if it’s a 1 you’re pregnant, if it’s anything between 2–6 you’re not. This is mainly just for fun though. If I were to determine fertility within the society (eg how fast the population of a city would grow), I’d use the lower figure of 4%.

PandoraBoxx's avatar

@DrBill, sperm can live for 5 days in cervical mucus, and there’s a three day window for ovulation, so I think your calculation is a little low.

grumpyfish's avatar

@Jack79 I think you’ve got it.

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