Surprise Pregnancy

Sometimes the Law of Inverse Fertility hits so hard that the woman doesn't even realize she's pregnant, often not until the Screaming Birth (and sometimes not even then). Cries of "But I Can't Be Pregnant" and "how could I have missed this?" ensue. For whatever reason, when they do notice before giving birth, it isn't the lack of periods that tips them off, but rather the morning sickness and Wacky Cravings. Heroines just don't keep track of their monthlies. Cue Babies Make Everything Better!

Not to be confused with But We Used a Condom, but it's likely to be a result.

WARNING! There are unmarked Spoilers ahead. Beware.

Examples of Surprise Pregnancy include:

Anime and Manga

Great Teacher Onizuka—Kunio Murai's Absurdly Youthful Mother didn't realize the implications of her sudden weight gain at first.
In Kodomo no Omocha Sana's biological Absurdly Youthful Mother mom Keiko actually didn't know she was preggers until she gave birth. Somewhat more understandable in the manga, where Sana's birth happened when Keiko was 14 (she's still very young in the anime, but not THAT much), and it's strongly implied she's also a Child by Rape.
This happens to Shigeru in Akkan Baby, though in her case it's not so much "But I Can't Be Pregnant" as "how is babby formed?"
This trope was played tragically straight in Berserk, where Guts and a now insane Casca only discovered that she was in her beginning stages of pregnancy after she miscarried their child under the severe trauma that she sustained from the Eclipse.

Fan Works

Used as a "plot twist" in way too many fanfics. It gets one conditioned to expect it any time someone is eating more than usual, puking, gaining weight, or craving odd foods in any medium, including real life.
- Though it is justified in the occasional fic where Mister Seahorse comes into play.

Film

The Monty Python movie The Meaning of Life, previous to the infamous "Every Sperm Is Sacred" song, featured a poor Yorkshire family whose mother was having babies drop right out from between her legs as she cooked and cleaned for her burgeoning family. This may also be implying that it's less that she didn't know and more that she didn't care, since they had dozens of children already.
The new cute swimming instructor in Water Boys discovers she's eight months pregnant and goes on a maternity leave immediately after managing to convince her five team members to throw away their pride and join her synchro-swimming team for her.

Literature

In a subversion, Maya Angelou relates that she knew she was pregnant in I Know Why The Caged Bird Sings, but the surprise was certainly on her family after she hid it from them for eight months.
Implied in The Fifth Elephant. Even if Lady Sybil wasn't surprised, her husband most assuredly was, and his stupid questions and her dry responses are particularly hilarious.
Daisy's mother in The Stone Diaries is unaware that she's pregnant.
Bluestar in Bluestar's Prophecy, thanks to her one-night stand with Oakheart. She is unaware of her pregnancy until another cat points it out.

Live-Action TV

This happened to Peggy Olsen in Mad Men.
Was a plot point on Eastenders a while back where a teenager gave birth not having a clue.
Happened to an obese woman in a first-season episode of St. Elsewhere.
The aptly-titled I Didn't Know I Was Pregnant is about this. See the "Real Life" category for details.
A fat woman complaining of severe cramps went into the ER and not only realized that she was pregnant but that she was giving birth to twins. Dr. Benton had to deliver the baby—something he "hadn't done since med school."
- Elizabeth Corday gets hit with this in the seventh season. Her response to the news was a flat "That's impossible."
Doctor Who Series 6 gives one to Amy Pond. The surprise element is made more plausible, so to speak, by having the Amy we see be a (non-pregnant) duplicate with her consciousness, while her comatose (pregnant) real body is elsewhere.
Happened in the latest series of "Whitechapel" when DS Miles' wife fears she might have ovarian cancer only for it to turn out she's 6 months pregnant
At the end of "Baby on Board", Modern Family‍'‍s third-season finale, Gloria tells us she had thought she was carsick but, no, she's really pregnant, as the camera shows her buying a pregnancy test kit.

Oral Tradition, Folklore, Myths and Legends

In the satirical 'legend' of Pope Joan, a woman went to Catholic school disguised as a boy and worked her way through the ecclesiastic ranks to Cardinal and finally Pope. Her cover was blown when she put her foot in the stirrup to mount her horse and promptly gave birth before a shocked audience.
A double-subversion from The Bible: Sarah has overheard three men (actually angels in disguise) telling her husband that he and Sarah will have a son. She laughs because she is old and has been infertile for a long time, and gets called out for laughing (and thus doubting the ability of God to make this possible) by the angels, which shuts her up. Sure enough, she does become pregnant, and she names her son Isaac.
- Isaac's name means "he will laugh" - his mother laughed at the prospect of having a child at her age, and boy was she wrong!

Theatre

In the stage play E/R (basis for the half-hour 1984 sitcom, not the hour-long 90's drama), a mother brings her teenage daughter into the hospital emergency room, complaining of "stomach cramps". The girl continues to deny she's pregnant even as she's giving birth.

Western Animation

In Family Guy, Peter looks up one of his old girlfriends, who's turned ugly and obese in the intervening years. Then a baby splatters out from between her legs, much to their mutual surprise.

Web Original

On The Guild, Clara was well aware that she was pregnant. Her husband, on the other hand, was quite shocked to find out.

Real Life

Truth in Television: According to The Learning Channel series I Didn't Know I Was Pregnant, women who tend to be a risk for surprise pregnancies often assume they were infertile, or think they used proper birth control, or have such irregular periods that they didn't notice the pregnancy, or are so ignorant about the signs of pregnancy that she mistakes them for an illness. Some women still experience spotting during pregnancy (and thus think that they're still getting periods), while others don't even get a baby bump—not even nine months in—or have a larger body shape that makes small(er) fluctuations in weight and build hard to notice.
On either the same show or something similar, there was a story about a larger woman who had been thought to be infertile and thus didn't notice any of the regular signs of pregnancy. When she went into labor, she had been suffering from a bad case of constipation and thought she was trying to pass a bowel movement. The description she gives of what follows crosses the line from Nightmare Fuel into Nightmare Nitro.
A teen magazine once had a real life story about an 18 year old girl who hadn't gotten her periods yet. She'd had a boyfriend and hadn't used protection while sleeping with him because she didn't know that ovulation happens before the actual period comes. One day, ages after she and the boyfriend had drifted apart, she got some pretty strong stomach pains on and off. Thinking it was her first period, she just stayed on the toilet and ended up birthing a baby boy into it. The baby was okay, but her mother had to take care of him for a few weeks before the girl snapped out of her denial and started taking care of him herself.