Who was traveling with a pregnancy test on Flight 815
Claire found Steve and Kristen's wedding plan book in the wreckage of the plane. The two were not only planning on getting married, but were also planning on getting pregnant.
It belonged to Kate judging by the worried way she says, "Who travels with these things?"
It belonged to Sun. She probably had doubts about being pregnant with Jae Lee. That's why she asked, nearly sure, if there was a pregnancy test, because it was hers and she knew it had to be there.
It was in the suitcase full of medicines given to Jack by Ethan.
It belonged to Cindy, she may be late or suspicious that she was pregnant; that could explain why she was taken by the Others.
Was Juliet's sister really pregnant?
Her pregnancy test did not show a line in the confirmation window. This suggests that the test may have been a false positive. Ethan was present in the building earlier, and Mittelos seemed to be willing to resort to drastic measures (her husband's accident) to get Juliet on the island. Ethan could have done something that would generate a false positive. The video of her sister with a child could also be a mis-representation. The child in the video could be adopted, or belong to someone else. We only have Ben's word that her sister's cancer returned as well.
Unlikely, since Juliet's sister immediately told her that she also made a blood test to be sure before telling Juliet she was pregnant.
Widmore is tied to one of the losties through a pregnancy test, this may be knowledgeable to the character it may not.