The period from onset of labor to the complete expulsion of the fetus, or to departure from the delivery suite. Sometimes, and especially in the US, it covers a much larger time period (viz., from 28 weeks of pregnancy to 7 days after birth and even 1 month after a full-term delivery).
See Fetus, Gestational age, Hypoxic-ischemic encephalopathy (HIE), Newborn, Perinatal focal damage, Pediatrics