A postpartum (or postnatal) period begins immediately after the birth of a child as the mother's body, including hormone levels and uterus size, returns to a non-pregnant state. The terms puerperium or puerperal period, or immediate postpartum period are commonly used to refer to the first six weeks following childbirth. The World Health Organization (WHO) describes the postnatal period as the most critical and yet the most neglected phase in the lives of mothers and babies; most maternal and newborn deaths occur during the postnatal period.[3] In scientific literature, the term is commonly abbreviated to Px, where x is a number; for example, "day P5" should be read as "the fifth day after birth". This is not to be confused with the medical nomenclature that uses G P to stand for number and outcomes of pregnancy (gravidity and parity). A woman giving birth in a hospital may leave as soon as she is medically stable, which can be as early as a few hours postpartum, though the average for a vaginal birth is one to two days. The average caesarean section postnatal stay is three to four days. During this time, the mother is monitored for bleeding, bowel and bladder function, and baby care. The infant's health is also monitored. Early postnatal hospital discharge is typically defined as discharge of the mother and newborn from the hospital within 48 hours of birth.