Many species are genetically compatible in the setting of Dungeons & Dragons, as the existence of so many hybrid races attests. As a general rule, unless there is a specific half-breed race listed, offspring from a union between compatible species are of the mother's race, though such offspring may also have physical features that are similar to that of the father thrown in. With permission, this might allow members of other races, for example, to take feats or use magical items and abilities normally reserved only for certain races.
As a note on crossbreeds, sterility is common in many, but there are also a great many crossbreeds that are able to breed true, their fertility unhindered. Whether or not your crossbred character is fertile or not, then, is up to your personal preferences. For percentages in any given population, here is the percentage chance that a given crossbreed will be fertile rather than sterile:
| Relation of Species | Percentage Chance of being fertile | Examples |
| Closely-Related Species | 75% | Half-elves, Half-orcs |
| Near Species | 50% | Half-dragons, Half-fey |
| Distant Species | 25% or less | Half-trolls, Half-minotaurs |
Fertility Check: To determine if a female becomes pregnant, three rolls are needed. The first is made by the female, and is a straight 1d20 roll, with the results of this role determining that female's level of fertility, as determined on the following tables, based on the mother's race:
Highly Fecund Race (e.g. goblins, kobolds)
| Roll on 1d20 | Fertility DC |
| 1-4 | Infertile |
| 5-8 | DC 20 |
| 9-12 | DC 15 |
| 13-16 | DC 10 |
| 17-20 | DC 5 |
Moderately Fecund Race (e.g. humans, orcs)
| Roll on 1d20 | Fertility DC |
| 1-5 | Infertile |
| 6-10 | DC 20 |
| 11-15 | DC 15 |
| 16-19 | DC 10 |
| Natural 20 | DC 5 |
Less Fecund Race (e.g. dwarves, elves)
| Roll on 1d20 | Fertility DC |
| 1-10 | Infertile |
| 11-15 | DC 20 |
| 16-18 | DC 15 |
| 19-20 | DC 10 |
Additional Modifiers and Rules:
-The use of the Fertility spell or related magic means that no roll is needed, as a female is instantly enhanced to her peak of ovulation.
-For races that are unable to crossbreed, no roll is necessary to indicate conception.
Conception Check: The second roll, to determine if conception happens, is made by the male, and is a straight Constitution check, with the DC of the check determined by the Fertility DC on the table above. This check is modified as follows:
-Highly Fecund Race: +2
-Moderately Fecund Race: +0
-Less Fecund Race: -2
Additional Modifiers and Rules:
-Because of the impassioned nature of such acts, male barbarians use their Raging Constitution on their roll, rather than their normal Constitution score.
-If the female achieves an orgasm around the same time as the male: +2 to the male's Constitution check.
-For more than one male partner: -1 to each male's Constitution check per additional male involved past the first one (sperm cells compete with each other).
-A properly-applied, fully-functional condom (usually made from latex or soft animal skin) will negate the need for a conception check, as will some magic in the E Spells section. However, a DC 7 Intelligence check is required to put on this birth control device properly. If this check is failed, the condom only provides a -4 to the conception check.
Acceptance Check: The third, final roll is made once more by the female, and is also a straight Constitution check, to determine if her body accepts the pregnancy. If this roll fails, then the pregnancy fails to take hold. The DC of this check is 10, with the roll modified as follows:
Highly Fecund Race: +2
Moderately Fecund Race: +0
Less Fecund Race: -2
Additional Modifiers and Rules:
-On a normal failure, a female's body simply does not accept a child, for whatever reason. Normally this means that the fertilized egg is expelled during the next menstrual period without further complications. However, if roleplaying the heartache of stillbirth is desired, this can be chosen by a player as a viable option, though hopefully only after significant consideration.
-On a critical failure of the Acceptance check (a natural 1), the life of the female is in danger due to the pregnancy, though she will not know this until the time birth comes around. During the contractions of birth, 4d4 temporary Constitution damage, due to blood loss and internal hemorrhaging, will be taken by the mother, which recovers as normal provided that she survives. A DC 20 Fortitude save will halve this damage, and a DC 25 Heal check will also halve the damage. If both checks succeed, then the damage is negated, though not the risk to the child. The offspring can only be saved with a DC 20 Heal check, administered immediately, or else the child will be stillborn. It is possible for a child to survive, but not the mother, or vice-versa. Childbirth, without magic or modern conveniences, is one of the most dangerous activities that a female can engage in.
-A Heal check can be made to replace the Acceptance check. A natural 1 on this Heal check, however, requires that the female roll the check instead, with a -4 penalty.
-Due to their perfectly-functioning bodies, many species, such as elven and fey females (plus related species, such as unicorns, gnomes, foxkin, and so forth), as well as those who have conceived from good outsiders, do not need to make the acceptance check normally, though it might be required in extreme cases with crossbreeding. Females of other species impregnated by elves or fey and related species get a +4 to their acceptance check.
-For cross-species matings, closely-related species cause no penalty to the acceptance check, near species cause a -2 to the acceptance check, and distant species cause a -4 to the acceptance check.
