[Cabin Fever (2002)] Karen was pregnant with Bert's baby the whole time.
Supporting Evidence:
1. When the group first pulls in to Old Man Cadwell's store, the song Wait For The Rain by David Hess is prominently playing on their car radio. Specifically, the point of the song that goes:
And the road leads to nowhere,
And the castle stays the same.
And the father tells the mother...
At this point, the radio is turned off and the next thing we hear is a line of dialogue from Bert. Although it's unclear who the line is directed at (it seems as if he's speaking to the group as a whole), he is facing Karen when he says it.
It's worth noting that Eli Roth (the director) specially contracted David Hess to make a new recording of Wait For The Rain for this film, so it's not so far-fetched to suggest the audio was deliberately arranged so that the lyric, "And the father tells the mother..." leads directly in to a remark from Bert.
2. When Bert returns to the cabin to find Marcy and Jeff dousing his unattended bonfire, Marcy angrily yells at him, "Can't you be responsible for anything?"
Her tone strongly suggests that there is history driving that outburst. She isn't just angry at him for almost starting a forest fire; she's angry at him for a history of irresponsibility that has caused grief to either Marcy herself, or others.
One plausible explanation for this could be that Karen has told Marcy that Bert has gotten her pregnant, and Marcy's angry at him for either carelessly getting Karen pregnant, for refusing to care for Karen and the child, for obviously being too immature to be an adequate father, or any combination of the above.
3. Paul has been pursuing Karen romantically for years, and yet all of a sudden she starts reciprocating his advances? Why now? Could it be that she has suddenly found herself in need of a stable, reliable man to take care of her and her child?
Paul is obviously much better husband & father material than Bert, and he's conveniently already smitten with her.
Perhaps it was Karen's intention to let Paul go all the way with her during the vacation, and then tell him that he'd gotten her pregnant.
4. While they are cleaning the truck together, Bert mocks Paul for constantly simping over Karen. Perhaps he (unlike Paul) is fully aware of the situation and finds it hilarious that Paul is about to be conned in to accepting responsibility for his child.
5. Karen is depicted as having a definite wild streak; during the campfire scene, she states that she was once drunk for five days straight, while hanging out with guys who wouldn't let her drink anything but beer.
Yet despite this, she is the only main character who is never once shown consuming alcohol during the film. Why? Is she abstaining because she's pregnant?
Problematic Detail:
1. Karen is shown smoking after the group arrives at Cadwell's store.
While this might suggest she isn't pregnant, it is worth noting that cigarettes are a lot more difficult to give up than alcohol. So she could still be pregnant; she just fell off the non-smoking wagon.
It's also worth noting that the first shot we see of her with a cigarette comes immediately after Paul tried to befriend the kid, Dennis.
Perhaps the sight of Paul acting all paternal towards a child was a hard-hitting reminder of the uncomfortable future she was facing (with Paul as the stand-in father of her child), and she just needed to light one up to take the edge off.
What do you think?