I really would not focus so much on variation without narrative meaningfulness.
I mean, what does birth weight have to contribute to things? Does it open up or close off different scenes and outcomes? Is there a narrative benefit to these mechanics?
I think that you might want to shift focus on "How does a pregnancy or childbirth contribute to our story?" instead of having an extra feature to implement just to say you have it.
Example: Hel's child in CoC can have several different outcomes, but the thing that really makes it a worthwhile thing to do is the extra character that you have more influence on than some statistics.
Another example: Ember can have a lot of variation by herself, but the real meat is how the alterations you make change her capabilities in addition to her appearance.