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Phenylalanine hydroxylase (PAH) deficiency is an inborn error of phenylalanine (Phe) metabolism that results in the buildup of dietary Phe to potentially toxic levels. Poorly controlled Phe levels in women of childbearing age are particularly worrisome due to the toxic effect of elevated Phe on fetal development. Pegvaliase was recently approved as an enzyme substitution therapy to reduce Phe concentrations in adult patients with PAH deficiency who have suboptimal Phe control on existing management. During the pegvaliase clinical trials pregnant patients were excluded from participation, but the approved label does not contraindicate its use during pregnancy. This case report describes the outcome of the first PAH deficient patient who elected to continue treatment with pegvaliase during pregnancy and reviews the lessons learned and future considerations.