I would imagine that there are birth weight statistics available someplace.
I haven't been around the neonatal wards much, but I would guess there could be a few issues that would be related to birth weight.
Some new treatments to prevent premature births.
I doubt subtle things like the fluctuating magnetic pole will affect birth weights. However, there could be changing natural selection. While mothers with heavy babies may induce labor early now, 100 years ago, a 10+ lb baby might have posed a significant risk to both the mother and the infant. Perhaps a historical search would show greater survivability of both the very heavy, as well as very light babies now than in the past. So, genes that would be related to issues that could put the mother of baby at risk would now be survivable, and passed onto the next generation.