The period from 28 weeks after conception through one week after birth.

The perinatal period spans the last stage of pregnancy and the first days following birth. Approximately coinciding with the last trimester of pregnancy, the perinatal period features the final development and maturation of the fetus. At the onset of the perinatal period, the fetus is fully formed, with eyelids that open and close, a detectable heartbeat, and has demonstrated detectable movement for about one month. During the perinatal period, the internal organs will complete their formation and development, and the fetus will go from approximately two pounds to its birth weight of seven to eight pounds on average.

A premature birth is one that takes place during the first two months of the perinatal period.