This is a bit more paperwork than just rolling a d20 every year, but I don't think it's too much to keep track of, especially since I've discovered the many joys of Google Sheets. I've been kicking it around for a little while. As usual, I'm looking for some feedback before I think about implementing this in my game.

First, wife generation.

When generating a wife character, you'll need to keep track of their constitution and their appearence. If it's just a one night stand with a faceless peasant character, use the PK's stats for childbirth instead of generating a new npc. In this case, Constitution is used to gauge potency and fertility. At CON 5, men become impotent and women become infertile.

Constitution : 3d6+2 = ~12, 5-20
Appearance: 3d6+2 = ~12, 5-20

***QUICK AND EASY CHILDBIRTH***
Average the couple's lowest appearance and constitution scores, and roll the result as a skill.

Crit: Conception, +5 to birth roll
Success: Conception
Failure: No Conception
Fumble: Miscarriage, -1 mother's CON

If there is conception, roll 2d6. Two 1's = carrying paternal twins. If twins, Roll 2d6 again. One 1 = Identical twins. Two 1's = Triplets. If triplets, roll 2d6 again. One 1 = identical triplets, Two 1's = quadruplets. Etc.

Next, roll the mother's CON. Add +5 if the conception roll was a critical, add +5 if her upkeep is superlative, add +3 if her upkeep is rich, subtract -5 if her upkeep is poor, subtract -10 if her upkeep is impoverished.

Crit: Easy Birth, child will require no survival rolls.
Success: Ordinary Birth
Failure: Difficult Birth, -2 CON, -1 APP
Fumble: Mother dies in childbirth, roll 1d2 to see whether the child lives or dies.



LONGFORM CHILDBIRTH

Step 1. Baseline Conception.

Take the average of the lowest partner's Constitution and Appearance.

Step 2. Frequency of Congress.
Modifies conception roll.

Roll 1d3

0. Never.
1. Seldom (A Few Times per Year): -5, Checkmark Chaste if only partner
2. Occasional (Every Month): -3,
3. Habitual (Every Week): +1
4. Frequent (Every Few Days): +5, Checkmark Lustful
5. Leporine (Daily): +10, Increase Lustful by 1

Raise one result if spouse.
Raise one result if spouse and Love (Family) passion is famous.
Raise one result if Love (Partner) passion exists.
Raise two results if Love (Partner) passion is famous.
Raise one result if one partner is 25 or younger.
Raise two results if both partners are 25 or younger.
Lower one result if one partner is 35 or older.
Lower two results if both partners are 35 or older.
Lower one result if Hate (Partner) passion exists.
Lower two results if Hate (Partner) passion is famous.
Lower one result if secret lover.
Lower one result if away for ~3 months of the year (garrison duty)
Lower two results if away for ~6 months of the year (garrison duty, court, and adventure)
Lower three results if away for ~9 months of the year (garrison duty, court, prolonged adventure or several adventures)
Lower one result if one partner is homosexual.
Lower two results if both partners are homosexual.

Add the modifier to the conception roll in step 3.

Step 3. Conception Roll.

Critical: Birth, +5 to Birth Roll
Success: Birth
Failure: No Conception.
Fumble: Miscarriage, Mother loses 1 CON.

Modifiers:
If trying to avoid pregnancy, subtract 1d6

Step 4. Birth Roll

Roll mother's CON, with a +5 for Superlative upkeep, +3 for Rich upkeep, -5 for Poor Upkeep and -10 for impoverished upkeep.

Critical: Extraordinary Birth
Success: Ordinary Birth
Failure: Difficult birth, mother lose 2 CON 1 APP
Fumble: Difficult Birth, mother dies in childbirth.

Step 5. Child Rolls

1d6 Extraordinary Birth:
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1 again, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.
2. Talented, take an additional Family Characteristic
3. Bonny, +6 APP
4. Vigorous, +3 STR, +3 DEX
5. Big, +3 SIZ
6. Invulnerable, +3 CON, no child survival rolls necessary.

1d6 Ordinary Birth
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.
2. Sickly, -1 CON, roll 2 survival rolls a year.
3-5. No effect.
6. Healthy, +1 CON, up to 1 reroll per year on the survival table.

1d6 Difficult Birth:
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.
2. Dim, do not gain family characteristic.
3. Infirm, -3 CON, roll 3 survival rolls per year.
4-5. Dies.
6. Roll again. If second 6, roll 1d6: 1) Dwarf Max 8 SIZ, 2) Hunchback -3 DEX -3 APP, 3) Mute, no skills that involve speaking 4) Deaf, roll awareness and all social skills at 1/2 their value, no music. 5) Poor Eyesight, roll Awareness, Recognize, Heraldry, and Reading at 1/2. 6) Albino, lose 1 HP for every hour of direct sunlight without proper coverings (Hood, cloak, gloves, full armor, etc.)

Here is what we have been using in our campaigns for a few years now:
http://nocturnalmediaforum.com/iecarus/forum/showthread.php?2155-New-Childbirth-Table-amp-Blessed-Birth&p=18243&viewfull=1#post18243

It has worked very nicely, too.


Some comments on your system...

1. Baseline
- If Fertility = CON, why is Appearance in there (also, see below)?
- Since CON is usually reasonably high for knights & Cymric characters (+3 CON) in particular, this will result in a big increase of fertility. How about using CON-5, which also nicely cuts CON 5 people off?

2. Frequency of Congress.
- Appearance should modify this, rather than the base conception rate, if Fertility = CON.
- Rather than rolling 1d3, I would let Appearance, events and the player to decide. If they want to go at it like rabbits, then hand out Lustful checks, while if they are more chaste, then Chaste checks would be appropriate.

3. Conception roll
- No issues here

4. Birth roll
- Penalty for a failure is a bit too steep, IMHO. How about just making it -1 CON?
EDIT: Also, I really, really dislike the Standard of Living modifiers here. Peasant women are having children left and right, and they are not all dying in the crib, etc. The -10 modifier here combined with the Difficult Birth pretty much ensures that not only do the peasant women start dropping dead like flies (due to the CON drop making Fumbles more likely, if the negative value comes to play), but also their offspring are weak and sickly and likely die as children. A follower such as a midwife could give bonuses to the birth roll, instead, although I think this is something that could be handwaved away, since many peasant women would have access to older women experienced in these matters, too.

5. Child rolls
- Extraordinary birth 1 should already be Twins without an extra roll. It is Extraordinary, after all. Heck, I would go ahead and make them identical twins without any more rolling. Very Myffic, as they say in Diskworld.
- Difficult birth looks a bit too harsh, too. Assuming a mother's CON 13, you have about 1/3rd chance of getting a difficult birth and now 1/3rd chance of the child dying in childbirth, too, which is 1-in-9 chance of a dead baby. Also, there is no good outcome here, unless you roll 1 an/or get multiples, which is again a bit counter to the general trend of a singleton being likely better off (bigger, etc). My suggestion, off the top of my head:

1d6 Difficult Birth:
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary. If a multiple birth, they are Sickly, see below.
2. - 3. No effect.
4. Sickly, -1 CON, roll 2 survival rolls a year.
5. Dies before, during or soon after the labor.
6. Roll again. If second 6, roll 1d6: 1) Dwarf Max 8 SIZ, 2) Hunchback -3 DEX -3 APP, 3) Mute, no skills that involve speaking 4) Deaf, roll awareness and all social skills at 1/2 their value, no music. 5) Poor Eyesight, roll Awareness, Recognize, Heraldry, and Reading at 1/2. 6) Albino, lose 1 HP for every hour of direct sunlight without proper coverings (Hood, cloak, gloves, full armor, etc.)

Very good points, I was conflating APP and CON in the longform conception because it seems like a good idea to do that in the shortform version, but it doesn't follow that the conception roll has to be the same across both versions. I also really let those conditionals to the congress table really get away from me.

I also agree that the -5 to CON on the conception roll is a good idea. This also makes what I had thought was necessary, -2 CON on a failed birth, unnecessary.

You're right that the effects of the difficult birth on the child are too harsh, I'll move that table down to the fumble result.

Step 1. Frequency of Congress
Roll the average of both partner's appearances. Modifies conception.

Leporine, +2d6
Crit: Frequent, +1d6
Success: Habitual, +1
Failure: Occasional, -1
Fumble: Scant, -1d6
Seldom, -2d6

Raise result by one if Lustful is famous, Lower result by one if Chaste is famous. Reverse if partner in question is homosexual.
Raise result by one if together all year, lower result by one level if separated majority of the year. (default is around a quarter)
Voluntarily raising the result by one level will earn you a check mark in lustful. Voluntarily lowering the result by one will earn you a checkmark in chaste.

Step 2. Conception
Roll the lower CON of the two partners, -5, + Congress Result.

Critical: Conception, +5 on Birth Roll.
Success: Conception
Failure: No Conception
Fumble: Stillborn, -1 CON

Step 3. Birth
Roll mother's CON, + Conception Modifiers, +5 if Superlative, +3 if Rich, -5 if Poor, -10 if Impoverished.

Critical: Easy Birth, +1 conception following year, Extraordinary Child
Success: Ordinary Birth, Ordinary Child
Failure: Difficult Birth, -1 CON, -1 APP.
Fumble: Mother dies in childbirth. Afflicted Child.

Step 4. Child Roll

Extraordinary Child
1. Identical Twins, roll 1d6 again for their shared result. If 1 is rolled again, Identical Triplets, roll 1d6 again. And so on.
2. Talented, take an additional Family Characteristic
3. Bonny, +6 APP
4. Vigorous, +3 STR, +3 DEX
5. Big, +3 SIZ
6. Invulnerable, +3 CON, no child survival rolls necessary.

Ordinary Child
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.
2. Sickly, -1 CON, roll 2 survival rolls a year.
3-5. No effect.
6. Healthy, +1 CON, if second survival roll is triggered, take the higher of 2 rolls.

Afflicted Child
1. Roll Again, if second roll is 1, twins are born. Roll again for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll again, if only one 1 is rolled, simply reroll it. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.
2. Dim, do not gain family characteristic.
3. Infirm, -3 CON, roll 3 survival rolls per year.
4-5. Dies during or soon after labor.
6. Roll again. If second 6, roll 1d6: 1) Dwarf Max 8 SIZ, 2) Hunchback -3 DEX -3 APP, 3) Mute, no skills that involve speaking 4) Deaf, roll awareness and all social skills at 1/2 their value, no music. 5) Poor Eyesight, roll Awareness, Recognize, Heraldry, and Reading at 1/2. 6) Albino, lose 1 HP for every hour of direct sunlight without proper coverings (Hood, cloak, gloves, full armor, etc.)

Much improved. A couple of points below.



Raise result by one if Lustful is famous, Lower result by one if Chaste is famous. Reverse if partner in question is homosexual.
Voluntarily raising the result by one level will earn you a check mark in lustful. Voluntarily lowering the result by one will earn you a checkmark in chaste.


The above two (Famous Trait & choice) should be combined. Otherwise someone who is Famously Lustful can get two bumps and that is a bit too much, IMHO, especially with that +2d6 (I'd just make it a flat +6).

(EDIT: In other words:
Choose Lustful or be Famously Lustful: bump the result up by one and get a check.
Choose Chaste or be Famously Chaste: lower the result up by one and get a check.)

Homosexuality should always lower the result. Makes no sense that a Chaste Homosexual man would be as eager to schtup his wife than a Lustful Heterosexual man.



Step 3. Birth
Roll mother's CON, + Conception Modifiers, +5 if Superlative, +3 if Rich, -5 if Poor, -10 if Impoverished.


I still have a major beef with the Standard of Living modifiers here, although given that the results are now slightly less drastic, it is not as huge a thing. I would be tempted to scale things down, though. -3/-1/0/+1/+3? Still an effect, but not as huge of one.



Fumble: Mother dies in childbirth. Afflicted Child.


My major problem with this is that the folklores are FULL of heroes and heroines whose mothers' died in childbirth, and they are not Quasimodos. Also, folklores are full of stories of babies being abandoned etc, for some fault, even though the mother survived (sometimes even by the mother).

Instead, I would simply roll 1d20 on Child roll and have like 5% chance of something going very wrong, leading to your 1d6 Afflicted Child result, although no need then for a separate twin result, since it will already be in the 1d20 roll. Something like:

1 = Roll on Afflictions Table.
2 - 4 = Sickly, -1 CON, roll 2 survival rolls a year.
5 - 16 = No effect. Ordinary child.
17 - 19 = Healthy, +1 CON, if second survival roll is triggered, take the higher of 2 rolls.
20 = Twins (or more). Roll 1d6 for each, if one lands on 1, they are the identical twin of the other, if both land on 1, triplets are born. Roll 1d6 again for each, if only one 1 is rolled, the original pair of them are identical and the third is fraternal. If two 1's are rolled, they are identical to the third child that didn't. If all three rolls are 1, quadruplets are born. Continue this pattern ad infinitum as necessary.

(Although I would simplify the Twins rolling, too... like:
20 = Twins (or more). Roll 1d20 again.
1 - 12 = fraternal twins, roll sex separately.
13 - 19 = identical twins, roll sex only once.
20 = Roll 1d20 again, but this time they are triplets (or quadruplets if you roll 20 again).
)

Afflictions Table
Roll 1d6.
1. Roll again. If second 6, roll 1d6: 1) Dwarf Max 8 SIZ, 2) Hunchback -3 DEX -3 APP, 3) Mute, no skills that involve speaking 4) Deaf, roll awareness and all social skills at 1/2 their value, no music. 5) Poor Eyesight, roll Awareness, Recognize, Heraldry, and Reading at 1/2. 6) Albino, lose 1 HP for every hour of direct sunlight without proper coverings (Hood, cloak, gloves, full armor, etc.)
2. Dim, do not gain family characteristic.
3. Infirm, -3 CON, roll 3 survival rolls per year.
4-6. Dies during or soon after labor.