pregnancy test
A small stick-like object used to determine whether or not a woman is pregnant, determined by matching (usually) pink lines after a urine sample.
In what tends to be uploaded to Danbooru, they're meant as a joke about unprotected sex and/or pregnancies unwanted by at least one of the involved parties.
A minor meme related to pregnancy tests cropped up in late 2014, where a feminine finger pointing to a pregnancy test would be added into existing images. (See Danbooru post #1824996 and its parent.)
Do not add the pregnant tag unless the character has a visibly pregnant belly.