...Y'all know what puberty is, right?
You can't just expect any sixteen year old girl to have a baby because they've reached an arbitrary age, they actually need to have developed enough to handle it. You need a robust body to even survive an attempt at pregnancy, let alone deliver a child successfully. That's true today, with our modern medicine and OB/GYN specialists and such, and it was certainly true in antiquity.
If anything I would argue that a sixteen year old capable of reliably and safely having a child (insofar as it's ever possible to safely give birth) would have to have been an early bloomer, though I'm no paediatrician so take that with a fistful of salt. I can, however, say with confidence that Estelle is on the other end of the curve, whether that be from malnutrition, mistreatment growing up, or just her own genetics.
She's small, is what I'm saying. Young.
Certainly I suspect the Duke would be ecstatic in his own way if she fell in love and gave him grandbabies, but not so much that he would push her into such a situation when she's not really ready. That kills people. A C-section is always an option, but it's not something one should count on before pregnancy gives you a few months deadline.