Perinatologist: An obstetrical subspecialist concerned with the care of the mother and fetus at higher-than-normal risk for complications. Since the perinatal period, depending on the definition, starts at the 20th to 28th week of gestation and ends 1 to 4 weeks after birth, a perinatologist logically could be a obstetrician or pediatrician but, in practice, a perinatologist is an obstetrician. The comparable area of pediatrics is neonatology. A high-risk baby might be cared for by a perinatologist before birth and by a neonatologist after birth. The word "perinatologist" is a linguistic sandwich of the Greek "peri-" meaning "around or about", "natal" from the Latin "natus" meaning "born" and "ologist" from the Greek "logos" meaning "student of."