Who's Your Daddy?

...and what does he do?

A woman finds out she is pregnant, and is uncertain which of her lovers (often a husband and a boyfriend) is the father of the baby.

This may resolve a number of ways:
 * The mother, upon learning the child's paternity, hides it so that her current partner will not leave her (or alternatively, so that the biological father will be safe from negative consequences.)
 * The mother's new partner, on discovering the baby is not his, abandons them, leaving her to cope with raising the child on her own. If the child is already old enough to remember this, this can be a pivotal traumatic childhood event.
 * The mother's new partner raises the child out of a sense of duty, but always resents the child and puts his biological children first.
 * The mother's new partner demonstrates what a good person he is by vowing to love and raise the child as his own.
 * The biological father, upon discovering his child's paternity, vows to love and raise the child.
 * The truth is never revealed, leaving everyone uncertain.

In modern times, DNA testing can confirm doubts. In older tales, Chocolate Baby or another Uncanny Family Resemblance may be the only way to resolve it.

A Sub-Trope of Mama's Baby, Papa's Maybe.

See also Gene Hunting, in which the child seeks his or her parentage. Nothing to do with the stock phrase used when one person taunts another in a fight.

Comics

 * This Married To the Sea comic.
 * Played with in Spider Girl, when a "new" Spider-Man shows up. After lots of hints that he's Peter's illegitimate son with ex-girlfriend Felicia Hardy, it turns out he's really.
 * Done with Lori Grimes from The Walking Dead. The truth is speculated on, but never revealed.
 * Jotaro in Usagi Yojimbo.
 * Spider-Woman: Jessica Drew initially refused to reveal who the father of her newborn son was; the only certainty is that it isn't Tony Stark (he was the only one who asked, and her reply was to dump a plate of food on him; clearly she has a reason to keep it secret). It's eventually revealed she got pregnant through artificial insemination, though whoever donated the sperm remains unknown.

Films

 * Freddy Krueger, technically. His mother was -- *shudder* -- gang raped by 100 maniacs.
 * The movie Father's Day.
 * In Local Hero, the protagonist asks a gang of punks whose baby is with them (the mother being the one female punk in town). They just look at each other.
 * The Miracle of Morgans Creek is an odd example: She bumps her head at a farewell party for draftees, gets married and pregnant in her temporarily poor judgment, and then never remembers who her husband is.
 * In the 1979 filmatisation of Hair (theatre), a major (and never resolved) mystery is who got Jeanie pregnant: Hud or Woof?
 * Considering Woof was holding the baby at the end, this troper always assumed it was him.
 * Buona Sera, Mrs. Campbell
 * The Tina Fey movie Baby Mama has rare example of who's your mummy thanks to IVF.

Literature

 * The Dark Tower. Was Susannah's baby from Eddie, or that incubus who raped her?
 * For added fun, the kid actually turns out to have TWO daddies
 * Has fairly major significance in the backstory of the fantasy novel Song in the Silence. The debate is not the heroine's, but her mother's -- since one of her lovers . The heroine decides not to go Gene Hunting, since she knows which of the men she thinks of as her father, and nothing else matters to her.
 * Bree's pregnancy in the later Outlander books is a major issue, since she lost her virginity to her caring, gentle lover Roger, and was raped by Smug Snake Stephen Bonnet in roughly the same period of time. Indirectly, Jamie's other child, William, has the paternity quandary as well.

Live Action TV

 * Every Soap Opera ever. Ever!
 * One particularly Egregious example: on One Life to Live, Nora intentionally got pregnant by Sam and pretended the baby, Matthew, was Bo's son. Then everyone found out he was Sam's son. Then it turned out he was really Bo's son. Then it turned out he was really Sam's son. Then it turned out he was really Bo's son. Sam is now dead, so the volley is probably over.
 * The Maury Povich Show once had varied topics, but nowadays seems to deal with nothing but women giving men paternity tests. Sometimes one woman will take up the entire show testing seven or eight men, find out it's none of them, and return another day. Of course, they could be making it up... Lord, please tell me they're making it up.
 * All things considered, would that really be an improvement for the poor kids stuck with these morons as parents?
 * Blair on Gossip Girl nearly had one of these in S1 (turned out to be a false alarm) and then does have one in S5.
 * The Jeremy Kyle Show does this a lot with DNA tests. "And the test reveals that John... (thirty second pause) ...is the father of Jean's baby!"
 * Lost: Sun had an affair before coming to the island, and had been told Jin was infertile. Thus Sun is not certain Jin is her baby's father until a sonogram in "D.O.C." shows the baby was conceived on-island (and Juliet reveals the island increases sperm count.) The issue seems to be settled, but some fans suspect Michael may actually be the father.
 * But in the episode "Ji Yeon",
 * Sue Ellen became pregnant on Dallas after an affair with Cliff Barnes. Turned out the baby was indeed JR's. And they did it again years later when Cliff bumped into old girlfriend Afton and her daughter. Despite Cliff's hopes this child too turned out to be someone elses daughter.
 * On The Secret Life of the American Teenager, there were two candidates for the father of Anne's son: George, her ex-husband and her new boyfriend. The boyfriend had been told he was sterile, but George had gotten a vasectomy years earlier, so everyone assumed that the boyfriend was, in fact, not sterile and the father. Then George confessed that he had lied about getting a vasectomy. This led the boyfriend to conclude that George was the father. The boyfriend broke up with Anne and left and that was that. There was a brief instant right after the baby was born where George wasn't sure if the baby looked like him, but that was quickly dropped. They never did a DNA test, but the show has made it clear that George is the father.
 * Hollyoaks did a rather convoluted plotline where the mother of the child had a serious relationship with the guy who WASN'T the father, but he was the only one who knew that he wasn't the father. Then the mother died and left him the baby, and he got together with the dead mother's sister (I know, c'est la Squick) and their relationship ended horribly. So then they had a big custody war over the baby because of the complicatedness.
 * on Battlestar Galactica Reimagined hid the paternity test results from her husband. It wasn't until after he learned of it and shared the results with the biological father.
 * Nip/Tuck has this with and the paternity of . After finding out that  is his father, due to a one night stand before  wedding to, she hides it from her husband. She reveals it to her son's father, her son, and her husband (in that order) causing her husband to kick her out. Her husband doesn't treat her son any differently and eventually forgives both his wife and his son's father for the affair.
 * The central concept of My Two Dads - the mother died without knowing who the father was, and a judge ruled both potential fathers had to raise the daughter together. Hilarity Ensues.
 * Note that the show took place before DNA testing was created. In fact by the end of the series, DNA testing has come out and they took one, but ended up throwing away the results without reading them, thinking it was better that way.
 * Another central concept of the trope - the FOX reality show Who's Your Daddy?, a show that makes Maury seem entertaining. A person who had been adopted as an infant must pick out his or her biological father out of a group of 25 men. It gained a lot of controversy and massive backlash and was pulled off the air after only one episode. The remaining 5 episodes aired on their reality cable channel.
 * A big part of the plot during season 8 of The X-Files. Though, being a paranormal show, and with Scully not actually supposed to be able to have children, it was both this and a 'How the heck did this happen?'
 * A subplot in Oh, Dr. Beeching, involved a new station master at a rural English train station finding an old flame running the canteen. She has a daughter who could be the new station master's, and it turns out that the mother was seeing both her husband and the station master at the same time. The paternity was never disclosed.
 * On Peep Show there were at least three candidates for the father of Sophie's baby—Mark, Jeremy, and Jeff. Mark has claimed fatherhood on the basis of Sophie telling him DNA tests have revealed it is his.
 * Shows up in Supernatural of all places. Dean meets an old flame who has a son who acts and looks like a mini-Dean and was born roughly nine months after they were together. The woman assures him that the son is not his and the real father left them shortly after her son was born. Some fans, however, theorize that she was lying, although this possibility is never addressed in-show. Dean eventually
 * The paternity of Aeryn's pregnancy became a prevalent source of angst during season four of Farscape. Thanks to Fantasy Contraception, Peacekeeper females can hold an embryo in stasis for up to seven cycles (years), so Aeryn had no way of knowing if the child was Crichton's until she made it to a medical facility near the end of the season.
 * This pops up with Claudia Black again in season 9 of Stargate SG-1. Vala, upon accidentally arriving in the Ori galaxy, finds herself pregnant with no explanation and gets married to avoid punishment from the incredibly religious townspeople. Vala and her husband do eventually find out that the child is "the will of the Ori", making Vala's pregnancy a rather dark take on immaculate conception.
 * Occurs in Call the Midwife with an unusually happy ending.

Radio

 * Happened on The Archers, when Emma wasn't sure whether her baby was Ed's or William's.

Theater

 * Mamma Mia!, in which there are three prospective men who could be Sophie's father...and all three are invited to her wedding.
 * And Hilarity Ensues!