It's not entirely clear how young the aliens are willing to go to select participants. The Pregnant Woman's fetus is treated as a separate participant when it falls into one of the participant circles, suggesting that there's no limit to how young the participants can be. Children younger than the Young Girl, maybe even infants, might have been selected for some circles. However, the aliens took the Pregnant Woman into the circle along with her fetus, rather than taking one or the other, suggesting that the aliens did not initially consider the fetus to be a separate participant, so perhaps they draw the line there. The fact that pregnant women are shown to be survivors of the circle while still carrying their babies suggests that, had the Pregnant Woman survived to the end, she would not have been forced into a vote against her own fetus.

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