A formal definition of normal birth has not yet been resolved upon in the US, although many segments of the birth community are actively campaigning for one!
Typically, in a "normal birth" labor begins spontaneously sometime between 37 and 42 completed weeks of pregnancy, and continues without intervention. Normal birth may involve:
Artificial rupture of the membranes if not part of medical induction
Nitrous oxide ("gas") — not currently available in our region
Intermittent electronic fetal monitoring
Managed 3rd stage labor (birth of the placenta)
Antenatal, delivery or postnatal complications including post-partum hemorrhage, perineal tear, repair of perineal trauma or baby admitted to a neonatal intensive care unit
The exclusion criteria of normal birth typically include:
Induction of labor with prostaglandins
Epidural or spinal anesthesia
Forceps- or vacuum-assisted delivery
Note the differences between normal birth and "natural birth" which occurs without medical management of any kind.
Normal birth, as defined by the World Health Organization, is a process that is spontaneous and continues without intervention. However, normal birth is no longer the norm for American women. Childbirth in the US is commonly treated as an illness and has gradually become more and more intervention-based. Reasons for this are complex, and influenced by cultural beliefs, education, technological development and litigation. For more information see the World Health Organization's Reproductive Health Library.