Unto Us a Son and Daughter Are Born
Twin characters in a work of modern fiction come in all sorts (though they have a disproportionate tendency to be identical). However, when twins are born over the course of the story (as part of a Babies Ever After or otherwise), they are almost always male and female fraternal twins. While this is common in Real Life, most actual fraternal twins are born in same-sex pairs. (Identical twins always are, of course; applying this trope to a pair of identical twins is a Critical Research Failure.)
Why does this happen? The birth of a boy and a girl instantly creates a fully formed family of father, mother, son, and daughter, putting all of the family relationship tropes into play, without any perceived bias by making the boy or girl older and in an authority position over the other. These twins are frequently the first and only children of their parents. Compared to other twins in fiction, it's more rare for these twin pairs to include an Evil Twin or be Separated at Birth. While they are occasionally Half-Identical Twins, more frequently one resembles the mother and the other the father.
This trope is less prominent in mythology due to the blurred lines between story and backstory. In ancient myths or works like the Silmarillion that are deliberately mythological, the birth of twin brothers is much more common. The birth of twin sisters is generally rare in all media.
Especially common for firstborn twins in a small Nuclear Family. If twins are born into a large family that already has more than one child of the other gender, this trope is more likely to be averted. When this trope is averted for other reasons it's a very bad sign, as one or both of the resulting children will usually be supernatural or evil. Mythological aversions tend to have different fathers or be destined to depose or kill each other. Aversions are usually still fraternal, though. It's rare to see infant identical twins in fiction, as they tend to appear fully formed at ages six to thirty.
A frequent trope in Soap Opera, although due to the sheer volume of twins in decades-long soaps this is also occasionally averted.
If the parents are the main characters and the twins are infants, it's Unto Us a Son and Daughter Are Born. If the twins are the main characters and the parents gave birth to them offscreen in the past, it's not. Childbirth scenes are usual but not absolutely required as long as the pregnancy and the twins as small infants are both shown.
- Basara: Tatara and Sarasa.
- In the manga adaptation of Mothy's Aku no Meshitsukai, the first chapter opens with the birth of the king's opposite-sex twins. This turns out to be a curse rather than a blessing, as before he was killed by the king, the fortune teller makes one last prophecy that the king's child will be born with a birthmark, a sign that he or she will bring the country to ruin. The queen ends up having twins, and it's the boy who gets the birthmark. The king orders that the prince be locked in a tower and kept there.
- Elf Quest: Suntop and Ember.
- Flash: The Tornado Twins, Don and Dawn Allen.
- And then Wally's kids, Iris and Jai West.
- Marvel Universe: Magneto's twins. Probably lots of others. Averted slightly with the Scarlet Witch's semi-imaginary evil demon twin boys.
- At the time the Scarlet Witch's twins were born (at which point they were not yet retconned as imaginary) the writer explained in an interview that he made them same-sex specifically to avert this trope.
- Spider-Man: Norm Osborn's twins with Gwen Stacy.
- Spawn: Wanda's twins.
- Archie Gets Married: Archie and Veronica have twins—Little Archie and Little Veronica. In the "Betty" version, the same thing happens, with Little Betty instead. Both are a case of Generation Xerox as well.
- Another Archie example; Archie's Sonic the Hedgehog has been shown having children with Princess Sally in a few alternate futures, and those children are always twins; a girl that resembles her mother, and a son resembling his father.
- In First Contact: A Gemini Story, Ferb has a twin sister that he was separated from him at birth named Jessica.
- Generally common in most Kid Fics.
- Jillian's children Tilly and Cj from Meg's Boyfriend
- Also, Jillian's children Kyle and Maya from The Spellbook
- Happens in the final arc of the Elemental Chess Trilogy (Fullmetal Alchemist). Played with in that when someone asks "Did she have a boy or a girl?" the answer comes back "No!"
- Common among the "Ragnarok Wave" in The Symphony of the Sword cycle of Undocumented Features. Lampshaded by Kozue Kaoru (herself the twin of Miki), who notes that her peer group has Guy and Priss Morgan, Chip and Reiyna Mui, and Achika and Tenchi Shannon -- all brother-sister sets of twins.
- Bambi: Geno and Gurri. Well, okay, they're deer. But still.
- Henry and Samantha Focker in Little Fockers, although their actual birth is never depicted.
- Luke and Leia at the end of the Star Wars prequel trilogy.
- A major plot point in Whale Rider. The boy was supposed to be the chief of his tribe, but he died soon after being born, leaving the sister with the weight of her grandfather's expectations.
- Animorphs: Darwin and Madra, the children (sort of) of Visser One.
- Possibly averted in the Belgariad with Polgara's twins, although since they're deliberately nameless and genderless it's really hard to say.
- Dune: Leto II and Ghanima. This one also subverts the usually-unplayed Twincest angle: Leto marries Ghanima, but it's for political purposes. It's not like he could have sex with her or anyone else for that matter anyway; his blending with the sandtrout made that impossible. Ghanima's children are actually and not-so-secretly fathered by Prince Farad'n Corrino, grandson of former Emperor Shaddam IV.
- O-Lan's twins in The Good Earth by Pearl S. Buck.
- Honor Harrington: Honor's siblings Faith and James.
- Little Women: Meg's twins, Demi-John and Daisy.
- Pegasus in Flight: Tirla's twins, Mischa and Miriam.
- Petaybee: Ronan and Murel.
- Occurs at the end of A Pirate's Pleasure by Heather Graham.
- Vorkosigan Saga: Aral and Helen. Perhaps not technically twins, but the same parents and born at the same time. (See Uterine Replicator for the It Makes Sense in Context on that distinction).
- The Bobbsey Twins: two sets—Bert and Nan are the older pair, Freddie and Flossie the younger. They're even Color Coded for Your Convenience: Bert and Nan are both dark-haired, Freddie and Flossie are both blond.
- In The Thrawn Trilogy, Leia gives birth to Jacen and Jaina; in Dark Empire she had another boy, Anakin. During the New Jedi Order, it was found that the Scary Dogmatic Aliens have superstitions about twins, so Shmi Skywalker's surviving descendants were able to take advantage of that. Tellingly, Anakin, the only single birth in the clan, was the only one who died there.
- Ramses and Nefret eventually have a son and daughter in the Amelia Peabody Mysteries.
- In the Women of the Otherworld series, the only female werewolf eventually gets pregnant, though this doesn't stop her from battling bad guys. When she finally gives birth, her children turn out to be a boy and a girl, and they're both werewolves (usually only sons inherit the trait).
- Averted in the Artemis Fowl series; at the end of The Lost Colony Artemis time-skips ahead three years and finds on his return that his parents have had twins, but in The Time Paradox we find out that the twins are both boys.
- In The Wheel of Time series, one of Min's viewings says Elayne's pregnancy will result in twins, a daughter and a son. As of The Towers of Midnight Min's viewings have never yet been wrong, so this trope will almost certainly come to pass.
- An interesting version in Sheri Tepper's Sideshow involves identical, conjoined twins who are hermaphrodites, but dressed as male and female in order to fulfil their parents' desire for this trope.
- Memory, Sorrow, and Thorn: Deornoth and Derra, Josua and Vorzheva's children.
- Naturally this happens from time to time in the Warrior Cats series, since having more than one kitten in a litter isn't out of the ordinary for cats.
- Varencienne gives birth to twins Rav and Elly in Chronicles of Magravandias. The boy looks very like her and the girl like the father. Played with in that Varencienne hated being pregnant and giving birth and had no intention of having more children unless she didn't have a boy.
- Dhattar and Aiony in the Firebringer Trilogy.
- In Sarum, a Neolithic chieftain's decades-long efforts to produce an heir eventually result in this trope. The "instant Nuclear Family" aspect is subverted, as a prophecy dictates that the daughter immediately be sacrificed at Stonehenge.
- Seer of Sevenwaters: Cathal and Clodagh's twins.
- As the World Turns had a double version of this with Jeff and Carol Ward's quadruplets.
- Brothers and Sisters: Tommy and Julia's twins, though one died.
- Coronation Street: Kenneth Barlow's twins, Judy Mallett's twins, Sunita Alahan's twins.
- The Cosby Show: Nelson and Winnie (Sondra and Elvin's children)
- Days of Our Lives: Sami and Eric Brady. Sami later gave birth to another set. And that's not even counting her twin siblings who were found in a meteor shower.
- A Country Practice: Simon and Vicky had twins, Tom and Charlotte
- Eastenders: Peter and Lucy Beale, Pete Beale and Pauline Fowler.
- Emmerdale: Cathy and Heath Hope.
- Friends: Chandler and Monica's adopted twins (a Babies Ever After).
- Averted in Full House, as Jessie & Becky's twins were both boys. Presumably the Olsens had the girl twin side covered, though they only played a single character.
- Get Smart: Max and 99's twins.
- Averted in Guiding Light, with Blake's twin boys, who were briefly thought to have different fathers.
- One Life to Live had this happen in the seventies with Larry Wolek and Meredith Lord's twins, though one was stillborn.
- Also happened in the 90's with Max and Luna's twins.
- Averted in 7th Heaven with Sam and David, the youngest of a large family.
- Also averted with the three sets of twin grandchildren - Matt fathered twin boys, Mary gave birth to twin girls, and Lucy miscarried twin boys.
- The West Wing: Toby's twins, Huck and Molly.
- Played straight on Little House On the Prairie when Nellie gives birth to twins. It's especially contrived since she and her Jewish husband had agreed beforehand to raise a boy Jewish and a girl Christian.
- The Nanny: Fran and Maxwell's son and daughter at the end of the series.
- Averted on ER with Doug and Carol's twins, which were both girls. Interesting, since the previous season, the audience had gotten a pretty strong hint that this trope would have been played straight (a psychic patient who claimed to be able to see inside people was trying to decipher if Carol was having a boy or a girl. Her expression seemed to indicate that it was both).
- Though not in The Bible, some Jewish sources say that each of Jacob's twelve sons was born with a twin sister. The same with Cain and Abel.
- Classical Mythology: Apollo and Artemis. A weird variant is Leda's quadruplets: Castor, Polydeuces, Clytemnestra and Helen.
- Where according to the most popular version Helen and Polydeuces were fathered by Zeus, and Castor and Clytemnestra by Leda's husband Tyndareus. From eggs, one for the brothers and one for the sisters. You see, Zeus had mated with Leda in the form of a swan and...you know what, this is Greek Mythology. Things aren't supposed to make sense.
- Egyptian Mythology: In some myth variants Osiris and Isis are twins.
- Norse Mythology: Freyr and Freyja. Borgny gives birth to a set in the Elder Edda.
- In the third act of Abie's Irish Rose, Solomon and Patrick, though still angry at each other, are eager to see Abie and Rose Mary's new baby. Solomon wants it to be a boy like his son Abie, and Patrick is eager for it to be a girl like his daughter Rose Mary. It conveniently turns out to be both, so the feud can easily be settled.
- In Of Thee I Sing, after the First Lady saves her husband from impeachment by announcing that he's about to be a father, the question on everyone's minds is: Will it be a boy or a girl? The answer, as decided by the Supreme Court: Both!
Wintergreen: That makes me a father and a mother. Twins! That's a little more than I counted on!
- In Dream Girl, when Georgina hears about her sister Miriam's pregnancy, she dreams of switching places with her and giving birth to twins which she names Gerald and Geraldine.
- Coga Suro: Towards the end of the first run, twins John and Mary are born.