"We do not mourn for fetuses, and anything which does not live for 30 days, we do not mourn for it."
Maimonides,
Mishneh Torah, Laws of Mourning 1:6
"The infant, for 30 days, even including the full 30th day (if it dies), we do not mourn for it."
Shulhan Arukh Yoreh De'ah 374:8
