Sex makes babies.
However, there is a certain lag-time between sex, conception and a positive pregnancy test. It takes hours or even days for sperm to fertilize an egg, and days more for the zygote to implant, which is the earliest a woman's body can detect pregnancy. A woman will most likely not even suspect she's pregnant until her first missed period. Not so in fiction, where plotlines require advancement and where women consequently can detect pregnancy within days or even hours of conception.
There can be variations on how she does it.
This is a subtrope of Artistic License – Biology
. The clues that she's pregnant are realistic biological clues, but they happen unrealistically early. Pretty much any physical symptoms before a missed period are examples of this type. One particular symptom, morning sickness, is very commonly used this way as a character's first sign that she's pregnant, even though in Real Life
it doesn't happen until at least six weeks after conception.
Type 2 is less tangible. The mother just knows
that she's pregnant, or just had a feeling
. This works in fantastic universes where there could be a magical or psychic reason for her to know. If it happens in a non-magical setting, chalk it up to a good guess with a hint of Women's Mysteries
. (We tend not to see the cases where a woman has sex, says "I just know I'm pregnant!" and then turns out to not
Anime and Manga
- In the finale of the Sailor Moon manga it is implied that Usagi was able to sense when her daughter was conceived.
- In a Thorgal story, after Kriss de Valnor is raped, her traveling companion Sigvald is able to figure out that she's pregnant when she says her breasts are growing bigger. However, there's no indication of how much time has passed since.
- Sil announces that she is pregnant to Arden, seconds after the act, in Species.
- Barely a day after having artificial insemination in The Backup Plan, Jennifer Lopez' character Zoe gets it on with Alex O'Loughlin's character Stan. During, she has to run over and throw up, and then announces she's pregnant. Stan says "I think it takes a little longer than that". She, of course, means from the AI - though being a day previous, she still shouldn't be able to know (or have morning sickness, for that matter) so this statement is a little ironic, touching on lampshading.
- In Look Who's Talking, the scene of Mollie in bed with her boyfriend cuts to the scene of Mollie throwing up in a sink at work. The implication is that this is her first sign that anything's amiss.
- In Tennessee Williams's play The Rose Tattoo, Serafina says she knows she's conceived when she feels her husband's rose tattoo burning on her breast.
- In To Sail Beyond the Sunset, Maureen tells Brian after their first round of no-contraception sex that she is now an expectant mother.
- In an episode of NYPD Blue, Bobby and Diane have sex, and she tells him that they just made a baby. She could feel it.
- Averted in the first season of 24, when Teri's pregnancy test comes up positive on the day she was sexually assaulted. She correctly reassures Kim that it has to be Jack's child, not the rapist's, because pregnancies can't be detected that early.
- The Sims games use morning sickness as the first sign that a woman is pregnant. She'll run to the toilet and throw up, and then walk around with a Thought Bubble that contains a pacifier and a question mark. "Baby?" To be fair, a Sim pregnancy only lasts three days. But the prevalence of this trope in other media means that throwing up the morning after sex (or Woo Hoo, in this case) is a nice piece of prepackaged shorthand for pregnancy.