Your challenge is to introduce one development that will improve (decrease) infant mortality in the pre-industrial world. We'll arbitrarily set that as beginning in 1800, for a nice round date.
Washing and sanitizing hands before delivery? That made a big difference OTL, though it was in an era of professional and semi-professional doctors and midwives, who saw more patients and so who had more opportunities to spread disease. It may not make as much difference if its introduced earlier, I just don't know.
IMHO, though, improving infant mortality probably means increasing child and adult mortality. Population pressure means more disease, war, and starvation.