How can K'Ehleyr get pregnant unintentionally, according to real world science? I don't know.
Easy, no use of protection in anyway shape or form.(No pill, condom etc) Although an injection was mentioned in (either Umbra or Strange Bedfellows) when Kasidy Yates fell pregnant accidentally BECAUSE Sisko forgot his. Now, if you're talking about 2010, it's still easily done, even if she were on the pill, some other medication could have cancelled it out.