Being too thin in late pregnancy is not a good thing for mammary development.

Greater mammary development, hence increased number
of cells that secrete milk, means more milk produced in
lactation. But what can be done to optimize mammary development
in late-pregnant gilts? It was recently shown that
body condition of gilts affects their mammary development.
A gilt that is too thin (12-15 mm backfat thickness at the P2
site of the last rib) on day 110 of gestation has less milk-secreting
tissue (parenchymal tissue) in her udder than a gilt
with 17 to 26 mm backfat. This difference was achieved by
feeding varying amounts of feed throughout gestation (1.30,
1.58 or 1.83 times the maintenance requirements). Such
findings are important to assist producers in maximizing
potential milk yield of first parity sows and demonstrate
that body condition must be considered. Feeding regime in
gestation therefore has an impact on subsequent lactation
performance of primiparous animals.