Wedding Enhanced Fertility

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 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. See also Babies Ever After.

Examples of Wedding Enhanced Fertility include:

Anime and Manga

In the thirdrd 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.

Literature

In Twilight, Bella and Edward conceive a fast-growing Fetus Terrible on their wedding night.

Live Action Television

In Torchwood, Gwen and Rhys even talk 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."

Web Comics

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.
Amber and Mike in Shortpacked. It turns out that it wasn't Mike's... eccentric means of proposing 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.