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These are one of those tough ones. It's b/w A and B and A is best
...in which more babies were born to women over the age of thirty
"it" refers to "the age of thirty"
Reconstruct the sentence and you have:
...in which more babies were born to women over the age of thirty than [to women] underthe age of thirty the portion italicized in brackets properly makes the comparison b/w the two group of women and the portion in blue is what replaces "it" to properly refer to the ones over the age of thirty and the ones under the age of thirty
in which more babies were born to women over the age of thirty than born under it
By repeating the verb, the comparison is b/w:
babies born to women over the age of thirty - to - babies born under [women over the age of thirty]
The portion in brackets illogically replaces "it" and indeed, the subject becomes "babies" instead of "women" as mentioned by fresinha12