Niddah 44 - When Do Laws of Purity Start
The laws of niddah can potentially start on day one: if a newborn girl discharges uterine blood, she is ritually impure as an adult woman who has a menstrual flow. She remains in that state for seven days, after which immersion in a mikveh renders her pure. The earliest she can become a zavah is on the tenth day: she enters the potential zivah days on day number eight, and three days of discharge then would render her a zavah.
A boy can become ritually impure as a zav also on his first day: a male becomes a zav through two or more discharges of special sort of gonorrheal emission. Both male and female infants can become a metzora (spiritual leper), or receive ritual impurity of a corpse on the first day of their lives.
A child of one day releases his mother from yibum (levirate marriage): if a man dies childless, his brother should marry his wife, now a widow, or release her with a formal procedure of removing a show and spitting, known as chalitzah. A child, even if it was born after his father's death, and even if it lived for a short time, makes his father not childless, and makes his mother free to marry anyone she wants. Conversely, if the child's brother died, that child binds his brother's widow with the yibum obligation, and she will have to wait till he becomes thirteen, to give her a halitzah. It is enough for them to co-exist in life for even one day.
Art: By Thomas Couture - A Widow