According to Freud's theory of psychosexual development, the stage affected in a newborn is the oral stage.

Oral Stage: This stage occurs from birth to about 18 months of age, where the infant's pleasure centers on the mouth through activities like sucking and biting.

Anal Stage: This stage follows the oral stage, from 18 months to 3 years, focusing on bowel and bladder control.

Phallic Stage: This occurs from 3 to 6 years, focusing on the genitals and the differences between males and females.

Latent Stage: This stage occurs from 6 years to puberty, characterized by dormant sexual feelings and a focus on social and intellectual skills.

References:

Freud, S. (1905). Three Essays on the Theory of Sexuality.

National Institutes of Health (NIH): Child Development Theories