When a woman conceives and gives birth to a male, she shall be impure for seven days (Lev. 12:2)
In principle, spiritual impurity is caused by the departure of holiness. A holy place or object becomes unclean when the sanctity that once rested on it is no longer there. (It is for this reason that a dead body is considered impure, i.e., the soul that animated it has departed.) Our Sages taught that only G-d holds the "key" to the miracle of birth; only He can "open" the womb or keep it closed. Every birth is therefore a manifestation of holiness, after which the sanctity departs, creating a state of spiritual impurity. (The Kotzker Rebbe)
Upon the completion of the days of her purity for a son or for a daughter (Lev. 12:6)
Up until the "completion of the days of her purity" the newborn is referred to as a "male" or "female"; only afterward does the Torah call the baby a "son" or "daughter." According to the Torah, 30 days must elapse before a neonate is considered out of danger. (Meshech Chochma)