Deuced if I can find it, but I thought there was a simple rule somewhere for rolling up conception. I found the rules for childbirth, no problem.

I am well aware that this can simply be a Storyguide fiat. I am asking if there is a RAW rule.

I'm not with my books right now but their might be something in the Fertility Magic chapter in (I am pretty sure) Hedge Magic. I know it expands upon the rules of pregnancy and childbirth and such.

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I do not find it in the Fertility Magic chapter of Ancient Magic, which I am sure you were referring to. Thanks, though.

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There is an article on Conception and Pregnancy in Sub Rosa #11. Midwifery stuff is in A&A, but I think you already found that.

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I did. It's also mentioned in Grogs.

If you have access to the Pendragon RPG book, it has rules for yearly childbirth within the family.
Alternatively, you can use modern data: 30% within the first month, 75% within six months, 90% within a year and 95% within two years.

So get your d20 out.

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Other than the “conception with magical aids” in Ancient Magic, Control Fertility from HoH:S (which just helps a person conceive) and maybe something under folk witches the only real rules around child conception I know of are those rules in Sub Rosa, so not canon. Never leads to gifted twins though which I found sad. :’(


I supposed something involving the Staminas of the would-be mother and father against a standard difficulty of 6 makes sense.

Now there's a new character Flaw: Fertile. (Flaw because it complicates the character's life.)

Lords of Men has rules for generating a (noble) family. You could use those to roll a future family as well...

There is a “virtue.” One of the sample lesser benedictions in RoP:Faerie (p114). It’s called “Unusually Fecund” and there is a related image that is both fantastic and illustrative on the same page.

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Useful, yes, but that's for a different sort of generation than I have in mind, where two characters in play, Man A and Woman B, love each other very much.

Fantastic is what she said.

Yes, someone has the "benediction" of unusually fecund and then has octuplets because... I would hope the benediction had some limitation for those who were already pregnant...
if we were to derive numbers from Lords of men there would be a range of 0-10 children in a 12 year span with a median value of 5.5, suggesting a roughly 50% chance every year of a pregnancy which will survive into adulthood- since roughly 30% of children died before age 5, this would mean there would be a child conceived on a 8 or less each year, probably round that up to an 9 or less to account for miscarriages which are in the rules on delivery. Then a 50/50 chance if it is male or female, and of course determine the birth month or season if you want that much detail.
Though with these rules you would likely need to make some sort of roll for childhood illness every year as well until the age of 5.

Rule proposal is

Sta F + Sta M + Age Modification against a difficulty of 9 (Stress). Stamina standing in for general health.Roll each season.

Age Mod, each:

To 25 +3

To 30 +2

To 35 +1

To 40 0

To 45 -1

Presuming fertile people and opportunity. I set the Dif at 9 because otherwise women would be getting pregnant every possible season, which seems a bit much.

Two ordinary average young adults, under 25, get busy:

0 +0 +3 +3 = 6 vs 9. On a roll of 3+ she is expecting. Most seasons, pregnant.

That may still be a bit too fecund, though it seems fairly consistent with the general observations about pregnancy, numbers of births, and so on.

Then I suppose three seasons of checks for complications. I'll have to review the pregnancy rules regarding carrying to term.

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I feel like the living conditions modifier probably should be added in, but then increase the difficulty by one step, to 12.

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The problem with the above is it has the same chance per season as should occur per year, if you raise the difficulty per season to 12 this would require a roll of 6+ in a season which is a 94% chance of pregnancy per year.

I agree that living conditions should factor in, which for most people is 0, but can be +2 for certain companions. I think this should either be the living condition of the mother or the less favorable living condition of the parents.

It's a good point about seasonal vs year probabilities. Considering the factors being ... uh ... factored in the Dif should be at least 12.

For a straight-average pair, the total is still +6 (0 Stamina, +3 each for age, 0 for living conditions).

For a favored pair, let us say a young upper class couple, Staminas of 1, +3 each, +1 living condition); 9 vs 12. Roll 3 to conceive.

For a poor, young couple living in a city with -1 Staminas, +3 each, -2 LC; 2 vs 12. Roll 10 to conceive. That's a heck of a swing in odds.


A lot of this depends on whether you are including the possibility of stillbirth in the odds to concieve- in which case the mother's health should matter but the father's not so much- possibly even excluding his constitution score

I'm not. This is not intended to be pregnancy rules. Those already exist. I'm just trying to set up a check for random conception.

Actually they don't. there are rules for delivery, but not for the possibility that a fetus will be conceived and then stillborn before it makes it to term, which was also common at the time, especially due to malnutrition of the mother (which is where the living standard really applies, and the father's doesn't matter to that)