A surefire way to detect pregnancy is puking in the morning, more efficient even than missing a period.
Never mind the fact that in Real Life, pregnant women may or may not get intense nausea which may or may not lead to vomiting which can happen at any time of the day (though The Other Wiki says that morning sickness is most common in the morning- not because it's a special time of day but because the nausea feels worst on an empty stomach). Of course the latter part isn't always used, pregnant women in fiction will often vomit at some random part of the day, often causing another character (often male) to comment, "Why do they call it 'morning sickness' if it happens any time of the day?"
Sometimes even animal characters will get morning sickness, despite it mostly being a human thing.
Because morning sickness is used so often as a tell for pregnancy, and because vomiting for other reasons tends not to come up as often in fiction, this causes expectations in the audience any time a female character is seen throwing up. If said character then goes on to speculate that the nausea must be stomach flu or food poisoning, that's as good as holding up a positive pregnancy test as far as most works of fiction are concerned. As a result, having the true cause of nausea be a surprise to the audience can be difficult to pull off.
Particularly egregious if it happens hours after the pregnancy is supposed to have been conceived — not only should no sign of pregnancy be detectable for over a week, but morning sickness should take about a month at least to manifest.
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Anime and Manga
This is what leads Sekai to suspect she is pregnant in School Days.
Same with Chizuru in the manga version of Bokurano.
CLANNAD ~After Story~: When Nagisa is throwing up in the sink, Akio good-naturedly teases her about possibly being pregnant. When Tomoya snaps at him about it, taunts him that if Tomoya was a man she should be by now. Cue Sanae cheerfully announcing "She's pregnant."
In Fruits Basket, we see Kyoko throwing up in the sink right before she tells Katsuyo that she's pregnant.
Subverted in the Fushigi Yuugi Eikoden OVA. Mayo is 3 months pregnant, and sits up in bed, heaving, thinking that it's just morning sickness (in her mind, a small price to pay for bearing Tamahome's child, especially in lieu of Miaka), but instead she vomits up a magical orb that allows her to see what Tamahome is up to.
A Jack Chick tract has a woman getting pregnant, introduced after she throws up several times during the day.
Subverted with Buffy in issue 5 of Buffy the Vampire Slayer season 9. She throws up several times, and together with some cryptic comments thrown her way it's the reason she goes out to buy a pregnancy test — which turns out positive. A few issues later it turns out she's actually a robot, and all symptoms are directly caused by this. The morning sickness for one is due to her artificial body's inability to process food.
Very, very, common in fanfiction, surpassing missed periods, early weight gain, and Wacky Cravings as early symptoms by a mile. The extent of the trope ranges from mild nausea to near constant vomiting from the day of conception. Not even men are safe from this trope.
Pregnant police chef Marge in Fargo says it's just morning sickness after inspecting a murder scene and almost throwing up.
Implied in Burlesque. Tess finds one of the dancers in the bathroom stall throwing up. She tells her, "Please don't tell me you have the flu." The dancer gives Tess a meaningful look, after which Tess says, "Please tell me you have the flu."
The book Clan of the Cave Bear, by Jean M. Auel, this is one of the first questions asked to verify that Ayla is expecting.
In Millicent Min, Girl Genius, the title character overhears her mother throwing up in the bathroom in the mornings. Being the girl genius of the title, with a superb memory and problem-solving skills above her peers, she carefully deduces beyond the shadow of a doubt... that her mother has terminal brain cancer.
Two characters in The General series, Whitehall's wife Suzette and 'Barbarian' princess Marie Welf are warned of inconvenient pregnancies by morning sickness.
Twilight, when Bella is on her honeymoon, about a day after her first day of sex. Of course it's a mutant baby growing ridiculously fast...
An attack of vomiting is what makes Anna realize she is pregnant in Jean Rhys's Voyage In The Dark.
In the novel The Microcolony by Gordon Williams, morning sickness was the first sign one of the micro-clone colonists was pregnant; something they didn't think was possible.
In The Parasol Protectorate by Gail Carriger Alexia Tarabotti gets nauseous on a zeppelin. Naturally it's later revealed that she's pregnant.
In Thrones, Dominations of the Lord Peter Wimsey mysteries, Harriet vomits several times earlier throughout the book, foreshadowing that She's pregnant with their first child, Bredon.
In the Temeraire novel Empire of Ivory, Captain Harcourt vomits several times before revealing she is pregnant, and continues to experience bad sickness throughout the pregnancy.
In The Destroyer, one of the villains "knows" that they've succeeded in having Remo impregnate them when they start having trouble keeping down their food seven days after having had sex.
Lanen in The Lesser Kindred. It's particularly bad because half-dragon embryos are very hard on the human body.
In Ice, Cassie experiences this. She initially chalks it up to food poisoning or illness, because she'd been taking birth control pills. When she finds out that her husband magically negated the pills, she was not happy.
Robert A. Heinlein's novel Friday. While on the starship Forward, Friday suffers from nausea. When she goes to the ship's doctor he insists on checking her for pregnancy before prescribing her an anti-nausea pill (c.f. Thalidomide), despite her protests that she hasn't had sex for a long while. The test comes back positive, much to her astonishment.
Honor Harrington's pregnancy is first hinted at when she feels slightly queasy at breakfast. Given her usual Big Eater tendencies, this stands out immediately.
Star Trek: Deep Space Nine reveals that Bajorans don't get morning sickness—instead, they have sneezing fits, usually five or more sneezes in rapid succession.
House, being the jerk he is, once mocked Cuddy with "Feeling a little sick this morning?" when he was suspecting that she was pregnant.
Pam on the American version of The Office (US) uses this to deliberately induce a Vomit Chain Reaction, to get back at her colleagues for not being sufficiently understanding of her sensitive condition.
In The X-Files Scully is shown only having very little morning sickness. And the one she does run for the bathroom, it's actually sometime in the evening.
Played with during the pregnancies on Charmed. Phoebe mentions that it should be called "Morning, Noon, and Night sickness," and feels awful (understandable, see below). Piper seems to play it more straight, being uncomfortable, but not quite as bad. Except when she throws up in her mouth at the hospital. Other symptoms in the Charmed world include attacking people with flowers (Piper's half-angel baby didn't like violence) and turning fruit into raw meat (Phoebe was pregnant with a half-demon.)
Played with in The Sarah Connor Chronicles: Sarah comes down with an illness and starts throwing up, which causes Cameron to suddenly ask, out of nowhere, if Sarah is pregnant.
John: Why the hell would you even ask that?
Cameron: Kacey vomited when she was pregnant. (to Sarah) You're vomiting. It's morning. That's when it happens.
In the 80s telenovela Rosa Salvaje, Candida is very nauseous at dinner, which prompts her vile sister Dulcina to taunt her about possibly being pregnant. Candida denies it, of course, but later finds out she is pregnant.
Happens to Yokas in Third Watch. Her partner Boscoe even gets the obligatory line about it happening all day.
In Doctor Who, Amy is sick during "The Impossible Astronaut," leading her to say at the end "Doctor, I'm pregnant!" Later revealed to be be an effect of the Silence. Which leads to Fridge Brilliance or Fridge Logic when it's revealed that the Amy we see is a plastic replica and the real (pregnant) one is somewhere else. The show is uncertain as to how long the season took and exactly when she had been kidnapped and replaced, but we do know that the replica body felt the contractions. Most likely the sickness was the same.
How I Met Your Mother averted this by having Lily be subjected to food poisoning. Then the twist at the end was that she actually wasn't. She was pregnant.
Gossip Girl had an episode dedicated to Blair's morning sickness. It was nowhere to be found the previous episode and went away completely in the following.
Delinda threw up on Danny in Las Vegas, then used the obligatory "happens all day" remark.
The smell of leather car seats did this to Gaby on Desperate Housewives, causing her to upchuck on the upholstery and royally decrease the vehicle's resale value.
The Stargate Atlantis episode "Be All My Sins Remember'd" starts with Ronon and John discussing how Teyla probably has the flu since she's been sick and looking "green." Later in the episode, she finally tells them she's three months pregnant. Different from most examples since Teyla found out she was pregnant before any external signs happened, but this was the first in-universe evidence for the characters, even though they didn't figure it out.
In The Sims 2, when a sim is pregnant she will run to the bathroom sometime the day after she conceives, and throw up. Afterward, she will have a thought bubble over her head with a pacifier and a question mark, wondering if she's pregnant. (The player always knows that she is—a lullaby chime plays after the couple "woohoos" to let you know.)
Sims 2 actually covers this semi-realistically; Sims can have morning sickness at any time of the day, and the severity of it varies from Sim to Sim. There are Sims who were only sick once, Sims who were sick several times in a row (resulting in a strange green gas coming from the toilet), Sims whose sickness waxed and waned, and Sims who simply felt nauseous.
Sims 3 is similar, the mother-to-be will randomly get a "nauseous" moodlet, and then usually throw up after a couple of hours.
In The Walking Dead, Christa experienced nausea when Lee dug up the dead dog, and she ended up vomiting. Lee also mentions she's been throwing up a lot. The fans of the game were unsure if she was pregnant of not, until "All that Remains" came out, when it showed her heavily pregnant arguing with Omid about baby names. There is 16 month time skip after Omid's death, and it shows a no longer pregnant Christa with Clementine, but no baby. It can be assumed the baby did not survive birth, as Christa would likely be dead if she had miscarried.
In Candi, a storyline, starting here and continuing for the next four strips, lampshades this trope and reveals that Candi is lactose intolerant.
In S.S.D.D an arc where Anne thought she might be pregnant started with her throwing up, after asking Richard if they were ever going to get married.
Happens in Shortpacked!. Willis managed to avoid having the readers catch on right away by giving a plausible explanation (nasty old pizza boxes) for the first bout of nausea.
In chapters 20 and 21 of We're Alive Lizzy has a tendency to start throwing up during stressful situations. She's revealed as pregnant in chapter 30
The Simpsons: Marge's pregnancy tests in different episode flash backs.
Homer: Back then, there was no way to tell if a woman was pregnant. (Gilligan Cut to Homer calmly walking to his bathroom, only to be forcefully pushed aside by Marge as she runs past; muffled vomiting sounds ensue)
In one episode of Family Guy, used to depict how typically clueless and self-centered Peter is, he flashes back to "all the difficulties" he had when Lois was pregnant. Cue him slumped on the bed, watching TV, while Lois loudly throws up and cries in the adjoining bathroom. He looks irritated, and turns the TV up.
This happened in Rugrats when it's revealed Didi is pregnant with Dil. They initially thought it was seasickness because they were on a cruise, and didn't find out until they visited the ship's doctor.
Not only can morning sickness occur at any time of the day, it can also happen if a woman is not pregnant. Some women even have the misfortune to experience this if a heavy period is screwing with their blood pressure.