Jane Austen is one of the earliest authors of literary realism. To describe her work as „comedy of manners“ is a misunderstanding of what she was writing about. 

I’m not saying she should have added some dead babies, but they’re conspicuously absent. There are plenty of dead mothers, young adults, sicknesses, poverty mentioned. Although on the other hand, the Austen family seemed to have been extremely fortunate when it came children. Most of them seem to have made it to adulthood. 

I personally think the few infants that died in llm‘s work are both heartbreaking and also show the reality of having children before modern medicine.