The Law of Inverse Fertility is a pain. If you somehow manage to not get pregnant when you are young and single and it would be disastrous for you to get pregnant, you will inevitably have great difficulty getting pregnant once you are married, settled down, and ready to have child. Isn't there any way to just have a nice, happy, non-angsty pregnancy?
Well... there is one way. But the bad news is, it is impossible to make it happen intentionally.
Wedding-Enhanced Fertility is when a female character has an unintentional but not unwelcome pregnancy the moment that marriage gets involved. It might happen immediately after the wedding, or it might happen while she's unmarried but her boyfriend proposes just before she tells him (if she tells him and he proposes because of it, it's a different trope). Both the parents-to-be are happy with the situation and the whole thing is treated as a pleasant surprise.
The confusing part about the trope is that there seems to be no narrative need for it. It is perfectly common for a couple to have children immediately after marrying, entirely on purpose, so the writer could just as easily have gone that route. Since the characters take the whole thing in a stride, there is no angst or drama to be found in either, after the initial shock. It seems that the writer just feels that while a pregnancy can be welcome and convenient, one thing it must not be is expected - essentially, Creating Life Is Unforeseen, even when done the old-fashioned way. See also Babies Ever After.
- In the 3rd Fushigi Yuugi OVA, the baby is conceived on Taka and Miaka's wedding night, making the baby extra special. It's actually a Living Macguffin needed to summon Suzaku and save the Universe of the Four Gods.
- In CLANNAD ~After Story~, Nagisa is pregnant a few months after marrying Tomoya. It does not end well, or at least not before the Reset Button Ending.
- In the For Better or for Worse fanfic The New Retcons, Elizabeth comes back from her honeymoon with Anthony pregnant. Unfortunately for all, it's not Anthony's baby. He drove her out of the wedding suite when he found out she wasn't a virgin, and she ran straight into Warren Blackwood's arms, leading to her son's conception.
- In Twilight, Bella and Edward conceive a fast-growing Fetus Terrible on their wedding night. Made either more or less astounding since it was the first time either of them had had sex at all, and they were under the impression that they couldn't have kids in the first place, what with Edward being technically undead and all.
- In Harry Potter, Lupin and Tonks elope soon after getting together, and she's revealed to be pregnant within a month or two. Whether it was this trope or a Shotgun Wedding is not entirely clear, though it was probably unintentional either way.
- In A Song of Ice and Fire, it's pretty obvious that Robb Stark was conceived on his parents' wedding night, since his father left for war the very next day.
- Edmure Tully and Roslin Frey conceive a child on their wedding night. While the child provides an heir for the Tully's he is conceived the night of the Red Wedding, the night where many members of the Tully and Stark families were murdered or captured by the Freys.
- In Slaughterhouse-Five, Billy and Valencia's post-sex conversation on their wedding night is interspersed with narration telling us that their son is currently being conceived.
- In Rainbow Six, Patsy spends most of the book pregnant, having gotten married to Ding shortly before the story began and conceiving during the honeymoon. Their son is born at the end of act two.
- Torchwood: Gwen and Rhys even talks about having children a few episodes before Gwen finds out she's pregnant, but the reason for it is apparently still that working for Torchwood puts your body through so many strange things that "the bloody pill doesn't stand a chance."
- Doctor Who 's River Song is conceived on her parents' wedding night - it is somewhat of a shock to the Doctor when he realizes that. Though the shock is to who her parents are: Rory and Amy.
- On Roseanne, Darlene proposes to her boyfriend David and only tells him that she's pregnant once he says yes.
- The last ever scene of The Mentalist takes place at Jane and Lisbon's wedding; the two sneak off to hang out by the lake while their friends enjoy the party, and Lisbon tells Jane she's pregnant - to his joy as well as her own - just before the credits roll. Despite a few very subtle hints in the preceding episodes that Lisbon is a little more emotional (read, in hindsight: hormonal) than usual, (and even this gets buried under her understandable reactions to Vega's death, Jane's disappearance after the funeral, and his sudden proposal upon his return), you'd be forgiven for feeling like it came out of nowhere. To be fair it does round off the series in a way, since it demonstrates that Jane's story has come full circle (the series started with him losing his first wife and child, and ends with him gaining a second wife and child), but the lack of build-up is slightly odd.
- Sherlock plays with this, as Mary got pregnant slightly before her wedding, but the two were engaged long before then and as such the baby had nothing to do with their decision to get married. In fact, her husband only finds out on their wedding day, shortly after the wedding, in a rather heartwarming scene.
- In Arthur, King of Time and Space, Modern-arc Guinevere finds out that she's pregnant right before Arthur proposes. Poor Fairytale-arc Guinevere is stuck with the Law of Inverse Fertility instead.
- In Shortpacked!, Amber realizes that she's pregnant right after Mike proposed. It turns out that it wasn't his...eccentric means of asking that made Amber throw up.
- Mac and Sam in Greystone Inn. Their honeymoon was to Disney Land, and apparently it's impossible to get hold of birth control there.
- Strongly implied in the final monthly Sabrina Online comic, which is a direct lampshaded Call-Back to Timothy's conception, which was a Shotgun Wedding instead.