Spoil the movie for me (Spoilers)

After barricading themselves in a house and shooting one of the children, causing the others to leave, the main characters (a husband and his pregnant wife) settle down for the night. The wife goes to sleep while the husband keeps watch with the gun.
During the night the wife wakes up screaming in pain. The husband thinks she's either in shock from the trauma they've been through, or is in labour. She seems to know instinctively that her unborn baby is, in fact, trying to kill her from the inside, having become prey to the phenomenon effecting all the children on the island.
The husband helps her to stand, and we hear the children laughing from the roof of the house. She explains that she let a young girl touch her stomach when they first arrived on the island (before they realized that the children were insane) and this must have infected the baby. Blood begins to run down the woman's leg and she collapses, dead, presumably from internal hemorrhaging.

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He left a note. He left a simple little note that said, "I've gone out the window."