A couple has had sex. They enjoyed it. There was afterglow. In the heat of the moment, nobody considered condoms or birth control. Or maybe they did but were in a bit of a hurry.
Then the woman announces her period is late, or starts every morning greeting John
Note that there are many reasons a period may be late or absent, ranging from menopause to diet or exercise programs to health issues of various sorts. But in this trope, she always jumps straight to "I'm pregnant".
She tells the guy who — in this trope — is almost never happy or ready to deal with this revelation that he may be about to be a father. They worry, they fret — separately or together — about what to do, and whether to consider abortion, adoption, keeping the baby, and what it will mean for their relationship in any of these instances. They agonize over the implications and whether they're ready to be parents. Will they be good ones? All the what ifs!
Then there's a pregnancy test — which comes up negative (or perhaps it was all some kind of huge misunderstanding). They are usually relieved, although there's sometimes a bittersweet melange of disappointment that there will be no unexpected blessed event.
If she's actually pregnant, it's not a scare; that's another trope altogether.
Often occurs after a one-night stand or between two people not in a relationship. The version where it's the man who is freaking out about it is common too.
On many occasions, when there's a pregnancy scare, it leads to discussion about the guy getting a vasectomy. But the vasectomy episode is another plot completely.
- Several years ago there was a series of ads for pregnancy tests which had three sets of couples:
- couple who want a baby and she's pregnant: YAY!
- couple who want a baby and she's not pregnant: AWW!
- couple who don't want a baby and she's not pregnant: WHEW!
- The "couple who don't want a baby and she is pregnant" version was never included in the ad.
- Happened to Silver Sable in the Marvel Universe during her own short-lived series. She thought she was pregnant after sleeping with her ex-husand and starting making plans to retire from the adventuring business. It turned out to be a glitch in her cycle and her doctor pointed out this was probably for the best as the reckless actions she had taken while she thought she was pregnant indicated she really wasn't ready to be a mother.
- In Grease, Rizzo and Kenickie's pregnancy scare is a major subplot. In this case, it averts But We Used a Condom, because Kenickie broke the condom while putting it on, and they do it anyway.
- In Birdman, the protagonist's girlfriend reveals to him that she missed her last two periods. His excitement is underwhelming which earns him a slap in the face. Later we learn that the girlfriend made her pregnancy up just to observe his reaction.
- In Preacher's Kid, Angie manages to get positive results on two pregnancy tests, making her think that Devlin has gotten her pregnant. Ultimately, it's revealed that she somehow "messed up" the tests.
- In Lois McMaster Bujold's novel Barrayar, Dru is very afraid that she's pregnant after sleeping with Lt. Koudelka. She doesn't tell him about it, but acts so nervous that he becomes convinced that he had been mistaken about her consent and had raped her.
- Happens to Tibby in the fourth The Sisterhood of the Traveling Pants book, after a condom accident. Though it wouldn't have been quite so bad if she hadn't been juggling multiple Idiot Balls during the whole thing. Without waiting for any indication that she was actually pregnant, she freaked out over the exact timing of when her period is due (is it one month after the last one began or after the last one ended?), and broke up with her boyfriend out of fear of the pregnancy.
- To Sail Beyond the Sunset: Happens to one of Maureen's daughters when her then-boyfriend refused to stop after the condom broke. She's not pregnant, her period shows up two days later, but it's still a valid concern — the daughter in question had been refusing to admit she was sexually active, so Maureen hadn't been able to give her any sex ed, and the pre-World War I era wasn't exactly accepting of unmarried mothers.
- Misfits: Seth finds a used pregnancy test — positive — and freaks out because he thinks he's gotten Kelly pregnant. He hasn't. Curtis has gotten his other-sex-self pregnant.
- Used on Switch (a short-lived sitcom in which husband and wife swap bodies); after something over a month, the husband in the wife's body has not yet had a period, and they worry that he's pregnant. He goes to the gynaecologist's office and hears all sorts of horror stories about how painful giving birth is. Turns out he's just overstressed, which causes irregular periods.
- On Gilmore Girls, Lorelai has a sudden craving for fresh fruit which hadn't happened since she was pregnant with Rory. She starts to think she might be pregnant by Luke but later finds out she is not. He never finds out about the scare.
- In Bones, Cam panics when she finds a positive pregnancy test in the lab's bathroom, considering that her teenage daughter had recently used it. She interrogates Brennan—not guilty—and realizes that it was Angela's. Who got a false positive, naturally.
- In Plain Sight: Mary hopes that her pregnancy will turn out to be this, even after the positive test. The doctor then gently reminds her that she'd have to be seriously ill to get a false positive.
- Saturday Night Live parodied pregnancy test commercials with a one-night-stand couple who really didn't want to be pregnant. Here it is on Hulu: 
- Part of a story arc in an episode of Roseanne between Dan and Roseanne. The scare ends up convincing them they actually do want to try for a fourth child.
- The The Golden Girls episode "The End of the Curse", Blanche believes that she's pregnant and spends the first part of the episode trying to work out who the father might be, when in fact she's entering menopause.
- On Grimm, Juliette has to have sex with Nick to restore his powers, but she has to do so while transformed into Adalind. Afterwards, she starts having headaches and stomach pains and Rosalee suggests she might be pregnant. It's not so much the possibility of pregnancy that worries Juliette, but the chance that the baby would have some connection to Adalind. It later turns out she's not pregnant. Instead, the ritual turned her into a Hexenbiest.
- M*A*S*H. A subplot of "What's Up Doc?" has Margaret believing she's pregnant after having spent R&R in Tokyo with Penobscott, and worries that giving birth will result in an automatic discharge. For confirmation, Hawkeye borrows Radar's female bunny for Margaret's pregnancy test, which results turn up negative; apparently, Margaret was simply having gallbladder problems.
- In the That '70s Show episode "The Pill", Jackie has a pregnancy scare. Donna starts taking the titular contraceptive, so the same thing wouldn't happen to her.
- On How I Met Your Mother, Robin is convinced that she is pregnant with Barney's child after she's late and "I'm never late!" In one of the most heart-wrenching mood swings, she finds out that, not only is she not pregnant, but she can never have kids.
- ER did this with Doug and Carol. True to form, after they learned that she wasn't pregnant, they realized that they were disappointed rather than relieved.
- Played for Laughs in this Fall On Me, where the female character lies about it to scare the male character out of his hiccups.
- In the "Anne's Pregnant?" arc of S.S.D.D Anne throws up at work the day after discussing the possibility of marriage with Richard. She takes a pregnancy test and while waiting both she and Richard are freaking out and thinking about how their lives could change. Then when the test is complete Anne asks Richard what he'd do if, hypothetically, she were pregnant, and after he faints and babbles about taking care of her she tells him the test was negative.
- Pokemon Pals: Misty has one of these after hooking up with someone she met at a bar.
- Sam & Mickey:
- "The Wedding" ends with Yasmin claiming to be pregnant with Ken's baby. In "Baby Shower", it turns out she was just feeling gassy.
- The Real Housewives of Toys R Us episode #2 has Barbie and Mrs. Potato Head worry that Barbie will have another baby soon, since after making out with Ken in an elevator, she appears to start developing Morning Sickness, Wacky Cravings, and Breast Expansion. Fortunately, when Barbie takes a pregnancy test (endorsed by herself), it turns up negative.
- The Simpsons: "I Married Marge": Marge thinks she might be pregnant. While she goes out to have a pregnancy test in the hospital Homer tells Bart and Lisa about the time Bart was born. After this Flashback Episode comes to an end Marge returns home to inform Homer that she is not pregnant after all, causing them both to do a high five.
- This is exploited by Hermes Conrad's wife LaBarbara in the Futurama episode "The Six Million Dollar Mon". When Hermes is depressed over getting fired, LaBarbara tells him that she is pregnant and that he isn't the father. After her husband panics, she assures him that she isn't pregnant and only said so to help him realize that losing his job isn't so bad when compared to an unexpected pregnancy.