The aqiqah is sunnah. The authority of Samurah that the Prophet sallallahu alayhi wasallam said: “The child is bonded by its aqiqah, slaughter for it on the seventh day, shave its head, and give a name”.
Its time begins at birth, and if done before birth it does not suffice. The preferred time is the seventh day. The day of birth is counted from the seven, and if a baby is born at night then the next day is the first of the seven.
What suffices for aqiqah is the same as that which suffices for udhiyah. The least of which is one complete sheep or goat or a seventh from a cow or camel.