Agree with raonx and spry. If we try to restore the full structure, it should be "more babies born to women over 31 than babies born to women under it.", it's really "babies born " that repeats, take second one out, it'll be "more babies born to women over 31 than to women under it". or it can even be simplified as "more to women over 31 than to women under it.".
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