Newborn animal or human infant during the first four weeks of postnatal life.
Note: For statistical purposes, some scientists have defined the period as the first seven days of human postnatal life. The precise definition varies from species to species.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 783 (https://doi.org/10.1515/pac-2015-1202)