Perinatal transmission: Transmission of a disease-causing agent (a pathogen) from mother to baby during the perinatal period, the period immediately before and after birth. The perinatal period is defined in diverse ways. Depending on the definition, it starts at the 20th to 28th week of gestation and ends 1 to 4 weeks after birth. The perinatally transmitted pathogen might, for example, be HIV (the human immunodeficiency virus) and transmission might occur across the placenta or in the breast milk. Also called vertical transmission.