Extra Parent Conception

A child is conceived, but instead of simply mixing one male sperm and one female ovum, a third set of genes becomes involved.

Most often used in a Free-Love Future or as a result of Bizarre Alien Biology, a child turns out to be the genetic offspring of multiple people—except, it was natural conception.

Perhaps we're dealing with aliens or supernatural beings and it's normal for them to conceive children this way. Or maybe, this was just some freak occurrence which took everybody by surprise. However it went down, Mass-Coitus Ensued and a baby happened.

Do not confuse with Homosexual Reproduction or Has Two Mommies, where a child is conceived or raised by two members of the same sex. Of course, this could overlap, if all parties were the same sex.

May be related to Polyamory or occur during A Party - Also Known as an Orgy. The situation may spawn (or should that be "conceive") an Aesop about such activities.

Examples of Extra Parent Conception include:

Comic Books

Sillage: One species requires one male and two females, though no details are given as to how it works.

Film

A Nightmare on Elm Street: Freddy Krueger is implied to have been conceived this way. His mother was a nun who worked at a mental institution where she became trapped alone with one hundred of the inmates over a period of several nights. Freddy was the result, and became taunted all his life as the "Son of a Hundred Maniacs".

Humor

A joke using this: two new parents stare at their newborn in disbelief, wondering how one child can have so many different attributes (sometimes they're listed: blond hair, black skin, small nose, huge ears...). Then they recall having participated in an orgy some nine months back. The baby starts crying, at which point the mother expresses relief that at least he isn't barking.

Literature

In Agent to the Stars by John Scalzi, the alien race who visits Earth can conceive children from arbitrary numbers of parents. The most prominent alien character, Joshua, is the child of the entire ship's crew, as well as the first human to visit the ship.
In Isaac Asimov's novel The Gods Themselves, the alien Soft Ones have three sexes, all of which are required for reproduction.
In the Gor series, the alien Kurii have three (or four) sexes, or genders. They basically work out to be male, female, a "blood nurser" which the females lay their offspring in and then the babies eat their way out of, and a fourth sex which is essentially a male with low testosterone, which is shunned by the community, but in certain circumstances becomes a breeding male; hence they're not sure whether to consider it a separate sex or not.
There is a story by Stanislaw Lem about aliens who, among other things, have five sexes.
Merry Gentry: Merry eventually gets pregnant with twins, and the twins have three fathers each.
A gag to this effect appears in the Rod Albright Alien Adventures series, when one alien mentions that normal honorifics and pronouns don't apply well to "him", because he comes from a species that requires five genders just to produce an egg, and three more to hatch it. Perhaps fortunately, he doesn't go into detail on this absurd arrangement.
The Culture by Iain M Banks: The Empire of Azad is ruled by a humanoids with three genders. Male, female and apex. An apex has a reversible set of equipment and carries the fertilized embryo from male to female. All three genders contribute genes, but the knowledge that females are more than passive child bearers is suppressed: the apices are very much on top and exercise crushing sexual discrimination against both other genders.
The Yeerks of Animorphs are sluglike aliens whose reproduction apparently involves three of them fusing into a single mass, from which larvae spawn.
Mr. Fusspot, the canine bank Chairman from Making Money, is metaphorically described as "the son of many fathers". His spoonhound mother got loose while in heat, and her owners never learned what male dogs she'd encountered before coming home pregnant.
In Slaughterhouse-Five by Kurt Vonnegut, the Tralfamadoreans reproduce with three genders, and further claim that humanity actually requires more than that to reproduce (although only two need to be physically performing the deed in three dimensions; the others are involved in an unspecified extradimensional way).

Live Action TV

The Newcomers in Alien Nation. Tenctonese have three sexes—male, female, and binnaum—all three of which are necessary to have a child. Because binnaums are relatively rare, they traditionally live monastic lives, only having sex when they are asked to help fertilize an egg for a male/female couple.
- One episode dealt with changing social roles as the Newcomers adapted to life on Earth. A binnaum has married a female and seeks the help of "his" male friend in reproducing (IIRC the binnaum was the one to help the male and his wife produce their own children years ago). The male is at first uncomfortable with this non-traditional situation (the male being an absent parent while the binnaum will raise the child), but eventually agrees.
In Earth: Final Conflict Liam Kincaid is the result of three biological parents: two human parents and one alien. Agent Sandoval, Agent Beckett and Ha'Gel, who was possessing Sandoval.
Zaphod Beeblebrox of the Hitch Hikers Guide to The Galaxy franchise has way more direct parents than anyone can count, due to an accident involving a contraceptive and a time machine. Because of this, his ancestors are also his direct descendants, and he shares at least three mothers with Ford Prefect, making them semi-half-cousins.
In the Star Trek Expanded Universe, Andorians have four genders, two that approximate male, two that approximate female. Since they have a low birth rate, it's a Hand Wave for why that race isn't seen much in space, despite being one of the Federation's founding species.
- And Species 8472 is known to have five different genders.
River Song in Doctor Who is Amy and Rory's daughter, but because she was conceived on the TARDIS, somehow has Time Lord DNA incorporated into her genetic makeup as well, prompting the Doctor to call her "the child of the TARDIS".

Music

The Mercedes Lackey song "Mis Conceptions" features a character who was apparently the result of a party with several mythological beasts as guests.

My mother never talks about that orgy
And I can't really blame her much, although
I would love to read the guest list for that party
And if there's another like it let me know

Mythology and Legend

The Epic of Gilgamesh: Gilgamesh claimed to be "two thirds god, one third human", implying he had three parents.
- To clarify, the Mesopotamians (and some other cultures) believed that if a woman slept with different men during or shortly before her pregnancy, that each of these men counted as a father. Thus the implication is that Gilgamesh's mom slept with at least two gods.
Heimdall, in Norse Mythology, has nine mothers.
Happens in Greek Mythology:
- Theseus was the son of both Aegeus and the sea god Poseidon, due to his mother taking an early morning wade in the ocean.

Video Games

The Grekim from the RTS Achron have three genders (octo, pharo, and sepi). They also have three 'classes' (basic, pod class, and ligo class). Any two different genders can mate to produce a member of the third gender of the same class or the next one up. (E.g. a pharo and a sepi can produce an octo or an octopod, or an octopod and a pharopod can produce a sepipod or a sepiligo.) This is actually the species' primary method of producing units in the game.

Web Comics

Uryuoms in El Goonish Shive reproduce this way. The egg can merge DNA from multiple sources, not necessarily including Uryuom at all. As they say (in the strip titled "Uryuom Sitcoms Are Insane"),

William: The current known record for number of parents to a single child is twelve, and only nine of those were Uryuoms.
Gillian: That kid needed some serious counseling.

In Homestuck, the trolls reproduce by mixing the genetic material of thousands of different trolls together within the womb of a Mother Grub, who then gives birth to hundreds of thousands of grubs. As such, they have no taboos against homosexuality or incest, and why they even have separate genders is unlikely to ever be explained canonically.
In Sluggy Freelance, the tri'ballians have 82 sexes, elaborate mating rituals and major interpersonal issues as a result. Their continued existence is a cross of A Wizard Did It and Rule of Funny.

Western Animation

In the Justice League Unlimited finale Epilogue, it is reveal that Terry Mcginnis is more than just the son of Warren and Mary Mcginnis, but the biological son of Bruce Wayne thanks to Amanda Waller of CADMUS secret acquisition of Wayne's DNA in an attempt to create a Batman successor. Waller had Warren injected with Wayne DNA while claiming it was a flu shot. That injection overwrote Warren's reproductive genes. This last-minute Retcon actually helps explain why Terry, and by extension his younger brother, Matt, have black hair while Warren and Mary have red hair.
- In reference to the spoiled content, this is also why they divorced. Two red heads will produce red-haired children, not any other colour.

Real Life

Polyspermy, in which two sperm penetrate a single egg simultaneously, can happen, even in humans. It's just incredibly rare, due to the egg's sperm-excluding cortical reaction, and the resulting triploid zygote cannot develop any further. Thus, a human Extra Parent Conception is theoretically possible, but a natural Extra Parent Birth is not.
There is an in-vitro fertilization technique where the nucleus of one egg is inserted into a different donor's egg, then fertilized and implanted. The Mitochondrial DNA of the donor egg would be from a third person.