American Academy of Pediatrics: Maternal phenylketonuria

Elevated maternal phenylalanine levels during pregnancy are teratogenic and may result in growth retardation, significant psychomotor handicaps, and birth defects in the offspring of unmonitored and untreated pregnancies. Women of childbearing age with all forms of phenylketonuria, including mild variants such as hyperphenylalaninemia, should receive counseling concerning their risks for adverse fetal effects optimally before conceiving. The best outcomes occur when strict control of maternal phenylalanine levels is achieved before conception and continued throughout the pregnancy.

Published by Pediatrics, 2001. The guideline itself is at the source linked here.

doi.org/10.1542/peds.107.2.427