BACKGROUND: Malformations and mental retardation in the offspring of women with Phenylketonuria (PKU) can be prevented by maintaining maternal blood Phenylalanine (PHE) within a target range (120-300 μmol/L) through a PHE-restricted diet. In a former French study, a high and unexpected proportion of intra uterine growth retardation (IUGR) has been reported. Guidelines have been proposed to all French centres caring for maternal PKU since 2002. OBJECTIVE: To confirm IUGR and investigate its causes. The other goals were to assess the follow-up of these pregnancies based on the new guidelines and the pertinence of these recommendations. DESIGN: Clinical, biological and ultrasound data of all pregnancies in PKU women in France, from 2002 to 2007 were retrospectively analyzed. RESULTS: Data from 115 pregnancies in 86 women with PKU were collected. Ninety percent of women had been informed of the risk of maternal PKU in the absence of a strict diet during pregnancy, 88 % of women had started a diet before conception, and 45 % of infants were born small for gestational age (birth length and/or weight ≤-2 SD). PHE intakes were lower in the group with IUGR from the fifth to the eighth month of pregnancy and duration of time spent at <120 μmol/L during pregnancy was associated with a higher risk of IUGR. CONCLUSION: Hyperphenylalaninemia (HPA) is not the only risk factor for IUGR; PHE lower than 120 μmol/L could also be associated with the IUGR occurence. Even if the monitoring of these pregnancies has been improved since the initiation of guidelines, we would like to stress on the importance of the dietary aspect of the disease.