I've met women shorter than that who have given healthy birth, and given that the father (of the baby) is also likely invested in both the well-being of both Natsuki and the baby, they would likely find a way to keep Natsuki safe from her father, probably in an abuse shelter of some kind, which will likely provide enough food to address her malnutrition. Lastly, 18 is a perfectly healthy age to begin having children, physiologically speaking. The reproductive system will have been active for a long time now. Most of the remaining pubescent development is psychological rather than physiological.

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