Phenylketonuria is an inherited, autosomal recessive deficiency of hep
atic phenylalanine hydroxylase activity. In affected individuals it is
manifest as hyperphenylalaninemia which, if not controlled, limits co
gnitive and neurological development. In pregnancy complicated by mate
rnal phenylketonuria, the developing fetus is placed at increased risk
of miscarriage or being affected by the maternal phenylketonuria synd
rome; a syndrome associated with intrauterine growth restriction, micr
ocephaly and cardiac malformation. Postnatal features include growth f
ailure, cognitive impairment, cerebral palsy and epilepsy. These adver
se effects occur regardless of whether the fetus itself does or does n
ot have phenylketonuria. In pregnancy complicated by maternal phenylke
tonuria it is possible to reduce the fetal risks by dietary restrictio
n of phenylalanine. Indeed, strong reassurance regarding the lack of a
dverse feta I effect from the maternal condition may be given if mater
nal serum phenylalanine levels are stabilized to within normal or near
-normal range, prior to and for the duration of pregnancy. A multidisc
iplinary approach to preconception and antenatal care is therefore req
uired in order to optimize obstetric outcome.