Chances of Getting Pregnant
Copyright (c) 1996 by MA Lloyd <malloy00@io.com>

Roll monthly (say with other housekeeping like collecting time use sheets).
Base chance of becoming pregnant is 6 or less, 3 is critical success.
Modifiers:
16-25 age group +1
35+ age group   -1
Good health     +1 (HT12+, or HT10+ and good diet)
Poor health     -1 (HT8-, or suffering an infectious illness that month)
Nursing         -2
Sex activity that month confined to
 a single week  -2 (assumed to be random, use +0 instead if you know 
                    ovulation happens then, e.g. ignoring a rhythm method
                    abstenance requirement)
 two weeks      -1
Using contraceptive method
 rhythm         -3 (requires abstinence one specific week in 4)
 pill           -4 (typical adherence to schedule)
 simple barrier -5 (condoms and diaphrams, critical success still bypasses)
Reliable contraceptive (implant, IUD or careful adherance to oral schedule)
requires a verified critical success (roll again on a 3, with no modifier
for contraceptive method).  Abstinence always works barring the activities
of a god with deviant sexual practices.