Maternal mild hyperphenylalaninaemia: an international survey of offspring outcome

The complete European guidelines on phenylketonuria: diagnosis and treatment

Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine metabolism caused by deficiency in the enzyme phenylalanine hydroxylase that converts phenylalanine into tyrosine. If left untreated, PKU results in increased phenylalanine concentrations in blood and brain, which cause severe intellectual disability, epilepsy and behavioural problems. PKU management differs widely across Europe and therefore these guidelines have been developed aiming to optimize and standardize PKU care. Professionals from 10 different European countries developed the guidelines according to the AGREE (Appraisal of Guidelines for Research and Evaluation) method. Literature search, critical appraisal and evidence grading were conducted according to the SIGN (Scottish Intercollegiate Guidelines Network) method. The Delphi-method was used when there was no or little evidence available. External consultants reviewed the guidelines. Using these methods 70 statements were formulated based on the highest quality evidence available. The level of evidence of most recommendations is C or D. Although study designs and patient numbers are sub-optimal, many statements are convincing, important and relevant. In addition, knowledge gaps are identified which require further research in order to direct better care for the future.

Key European guidelines for the diagnosis and management of patients with phenylketonuria

We developed European guidelines to optimise phenylketonuria (PKU) care. To develop the guidelines, we did a literature search, critical appraisal, and evidence grading according to the Scottish Intercollegiate Guidelines Network method. We used the Delphi method when little or no evidence was available. From the 70 recommendations formulated, in this Review we describe ten that we deem as having the highest priority. Diet is the cornerstone of treatment, although some patients can benefit from tetrahydrobiopterin (BH4). Untreated blood phenylalanine concentrations determine management of people with PKU. No intervention is required if the blood phenylalanine concentration is less than 360 μmol/L. Treatment is recommended up to the age of 12 years if the phenylalanine blood concentration is between 360 μmol/L and 600 μmol/L, and lifelong treatment is recommended if the concentration is more than 600 μmol/L. For women trying to conceive and during pregnancy (maternal PKU), untreated phenylalanine blood concentrations of more than 360 μmol/L need to be reduced. Treatment target concentrations are as follows: 120-360 μmol/L for individuals aged 0-12 years and for maternal PKU, and 120-600 μmol/L for non-pregnant individuals older than 12 years. Minimum requirements for the management and follow-up of patients with PKU are scheduled according to age, adherence to treatment, and clinical status. Nutritional, clinical, and biochemical follow-up is necessary for all patients, regardless of therapy.

Metabolic and mitochondrial effects of antiretroviral drug exposure in pregnancy and postpartum: implications for fetal and future health

Antiretroviral drugs (ARVs) are indispensable in the treatment and prevention of human immunodeficiency virus infection. Although their use before, during and after pregnancy is considered safe for mother and child, there are still lingering concerns about their long-term health consequences and the ramifications of their effects on lipid, glucose, intermediary and mitochondrial metabolism. This article reviews the known effects of ARVs on macromolecular and mitochondrial metabolism as well as the potential maternal, fetal, neonatal and adult health risks associated with abnormal energy metabolism during gestation. Recommendations about enhanced monitoring for these risks in affected populations are being provided.

Generation n + 1: Projected numbers of babies born to women with PKU compared to babies with PKU in the United States in 2009

Newborn PKU screening has been available since the mid-1960s, and the first group of screened babies is now a complete reproductive cohort (age 15-44). Untreated maternal PKU (MPKU) often results in significant developmental and physical disabilities in exposed fetuses, and could potentially offset some or all of the benefits produced by newborn PKU screening and dietary treatment. Based on the age distribution of the United States population in 2009, and using different estimates of PKU frequency (1/10,000; 1/15,000; 1/20,000), the projected number of babies born to women with PKU was compared to the projected number of babies born with PKU. In 2009, there were about 62,000,000 women age 15-44, with a fertility rate of 66.7 births/1,000 women. Of these women, depending on the incidence of PKU, 3,097-6,195 were estimated to have PKU, and they would have delivered 207-413 babies. In that same year, the number of births was 4,118,055, which would have resulted in 206-412 babies with PKU. Thus, in the United States, at all estimates of PKU frequency, the number of babies exposed to MPKU is equal to the number of babies born with PKU. This ratio varies with the fertility rate but is not dependent on the incidence of PKU. The benefits of newborn PKU screening and treatment could be significantly curtailed if adequate resources, education, and funding are not available to follow and monitor women with MPKU and their babies.

Non-PKU mild hyperphenylalaninemia (MHP)--the dilemma

Recent reviews have suggested that some patients with "non-PKU mild hyperphenylalaninemia" (MHP) might display neuropsychological executive function deficits and should be considered for treatment with tetrahydrobipterin (BH4) and/or phenylalanine (Phe) restricted diet. Patients with phenylketonuria (PKU)--Classical and Mild/Atypical variants--appear to need "mean lifetime phenylalanine (Phe) levels" of 120-360 μmol/L for optimal results. MHP patients, on the other hand, have natural Phe levels of 200-600 μmol/L. Until recently this was thought to be a benign condition. The available literature has been reviewed in detail and no good evidence, to date, has been uncovered to support treatment of MHP. It is suggested that more MHP subjects be tested to confirm this. A plea is made to formulate a consistent world-wide classification of the PKU phenotypes.

Impact of neonatal protein metabolism and nutrition on screening for phenylketonuria

OBJECTIVES

Early blood phenylalanine (Phe) elevation after birth enables screening for and anticipation of the diagnosis of phenylketonuria. The differential impact of factors involved in this phenomenon, however, has not been elucidated. To solve this question, phenotype, genotype, dietary Phe intake, timing of blood collection, and Phe metabolism were retrospectively analyzed in 21 phenylketonuria newborns and prospectively in 1.

PATIENTS AND METHODS

Patients were assigned to 1 of 4 classes of phenylalanine hydroxylase (PAH) deficiency (severe, moderate, mild, and benign) on the basis of their Phe tolerance. Phe ingested, tolerated, and released from endogenous catabolism was assessed.

RESULTS

From birth to screening test, the amount of Phe tolerated ranged from 704 to 1620 mg, according to the class of PAH deficiency. The amount of Phe ingested ranged only from 204 to 405 mg, whereas the endogenous Phe breakdown ranged from 812 to 1534 mg, resulting in a rate of Phe catabolism ranging from 262 to 341 mg/day, regardless of the class of PAH deficiency.

CONCLUSIONS

The high rate of protein catabolism is the main determinant of neonatal hyperphenylalaninemia. It is sufficient to turn to positive the screening test in severe and moderate PAH deficiency. In mild and benign PAH deficiency, the outcome of screening procedures can be substantially altered by the concurrence of genetic and peristaltic factors. These results imply that the value of blood Phe at the screening test is not fully predictive of the phenylketonuria phenotype, and strengthen concerns regarding the reliability of early screening procedures.

Maternal phenylketonuria syndrome and case management implications

Well-established dietary protocols have prevented mental retardation for infants born with phenylketonuria (PKU). Dietary protocols for managing females with PKU in their reproductive years exist but are not followed by most of them. Infants who are born to mothers with PKU who are not on dietary treatment usually have serious medical problems, such as mental retardation, heart defects, and other serious congenital anomalies (e.g., orofacial clefting and bladder exstrophy)--a condition known as maternal PKU syndrome. The focus of this article is to review the pathophysiology, associated developmental issues, and existing management protocols used to manage these two separate but highly connected disorders.

Maternal Phenylketonuria Collaborative Study (MPKUCS)--the 'outliers'

Analysis of outcome data from 305 of the 414 offspring from the Maternal Phenylketonuria Collaborative Study (MPKUCS), plus 70 control offspring, revealed significant deficits in the IQ (intelligence quotient), as measured by the Wechsler Intelligence Scale for Children--Revised (WISC-R), when maternal metabolic control during pregnancy was delayed and/or inadequate. There were, however, 23 'outliers' (7.5% of the 305) in which the offspring's intellectual IQ was worse (n =10) or better (n =13) than expected. The aim of this study was to determine whether collection parameters were incomplete or whether these subjects were true biological variants influenced by other undetected factors or, perhaps, by modifier genes. Among the 10 offspring whose intellectual functioning was worse than expected, additional complications were uncovered that could explain the poor outcome. Four of the 13 offspring with higher than expected IQ had mothers with mild variants of PKU in which the insult to the fetus would not be expected to be as profound. For the other nine offspring whose intellectual performance was better than expected, there was no explanation, based on the parameters studied. We hypothesize that modifier genes will, at times, protect the fetus despite high maternal concentrations of phenylalanine. Not all offspring from the same (untreated) PKU mother may be similarly affected. Finding the source of these modifiers might effect the treatment of MPKU.

Maternal phenylketonuria in Western Australia: pregnancy outcomes and developmental outcomes in offspring

OBJECTIVE

To examine the outcomes of phenylketonuric (PKU) pregnancies in Western Australia including birth characteristics and cognitive and behavioural outcomes in offspring.

METHODS

A cross-sectional study of women and their offspring who were identified from the Western Australian Maternal PKU Program (WAMPKUP) from 1991 to 2000 was carried out. Cognitive assessments (K-BIT or Griffiths scales) were conducted on women and their children, and behavioural assessments (CBCL) were conducted on the children.

RESULTS

Thirty pregnancies by nine women were registered on the WAMPKUP between 1991 and 2000. There were 16 live births, with one preterm delivery at 32 weeks. There were no congenital abnormalities. Five of the nine mothers and their nine children (aged 18 months-10 years) participated in developmental assessments. A linear relationship was shown between lower maternal IQ scores and later attainment of metabolic control in pregnancy (rs = -0.828; P = 0.01). There was significant correlation between lower offspring IQ scores and later attainment of metabolic control in pregnancy (rs = -0.734; P = 0.02). Correlation between maternal and offspring cognitive scores was not significant. Four of nine (44%) children rated in the clinical range for behavioural problems. Compared to children with no behavioural difficulties, these children had lower cognitive abilities (P = 0.05) and maternal metabolic control during pregnancy was poor (P = 0.05).

CONCLUSIONS

Poor metabolic control in pregnancy is associated with poorer cognitive outcomes and increased behavioural difficulties in offspring of mothers with PKU. The results have implications for the implementation of appropriate dietary measures before conception in PKU pregnancies, and indicate a need for the establishment of multidisciplinary teams to follow up individuals with PKU to communicate the importance of pregnancy planning, to manage PKU pregnancies, and to follow up the offspring.

Danger of high-protein dietary supplements to persons with hyperphenylalaninaemia

A 16-year-old adolescent with mild hyperphenylalaninaemia was given a high-protein 'body building' supplement twice daily, causing headaches, decreased school performance and mild depression. All symptoms disappeared after cessation of the supplement. The phenylalanine hydroxylase mutation H170D/IVS1nt5G>T was found to be responsive to tetrahydrobiopterin with significant decrease in blood phenylalanine concentration and increase in tyrosine blood content. A brain phenylalanine level of 0.5 mmol/L was initially documented, which decreased to the normal carrier range of 0.2 mmol/L within one month of discontinuance of the protein supplement. At present, the patient is on a normal diet without phenylalanine restriction.

Normal clinical outcome in untreated subjects with mild hyperphenylalaninemia

ABSTRACT There is international consensus that patients with phenylalanine (Phe) levels <360 microM on a free diet do not need Phe-lowering dietary treatment whereas patients with levels >600 microM do. Clinical outcome of patients showing Phe levels between 360 and 600 microM in serum on a free nutrition has so far only been assessed in a small number of cases. Therefore, different recommendations exist for patients with mild hyperphenylalaninemia. We investigated in a nationwide study 31 adolescent and adult patients who persistently displayed serum Phe levels between 360 and 600 microM on a normal nutrition with a corresponding genotype. Because of limited accuracy of measurements, Phe levels should be looked on as an approximation, but not as an absolute limit in every instance. In addition to serum Phe levels, the assessment program consisted of comprehensive psychological testing, magnetic resonance imaging of the head, (1)H magnetic resonance spectroscopy, and genotyping. We found a normal intellectual (intelligence quotient, 103 +/- 15; range, 79-138) and educational (school performance and job career) outcome in these subjects as compared with healthy control subjects (intelligence quotient, 104 +/- 11; range, 80-135). Magnetic resonance imaging revealed no changes of cerebral white matter in any patient, and (1)H magnetic resonance spectroscopy revealed brain Phe levels below the limit of detection (<200 microM). In the absence of any demonstrable effect, dietary treatment is unlikely to be of value in patients with mild hyperphenylalaninemia and serum Phe levels <600 microM on a free nutrition, and should no longer be recommended. Because of a possible late-onset phenylketonuria, Phe levels of untreated patients should be monitored carefully at least during the first year of life. Nevertheless, problems of maternal phenylketonuria should still be taken into account.

Undiagnosed maternal phenylketonuria: the need for prenatal selective screening or case finding

OBJECTIVES

The aims of this article are to report on a review of cases of maternal phenylketonuria in the International Maternal Phenylketonuria Collaborative Study that were initially diagnosed during or after a pregnancy, to alert health care practitioners to the possible existence of women with undiagnosed phenylketonuria whose fetuses are at risk, and to emphasize that not all adults with untreated phenylketonuria are mentally retarded.

STUDY DESIGN

The study was conducted through retrospective database review.

RESULTS

Of 414 women with live-born infants, 17 fulfilled our criteria. Six had phenylketonuria diagnosed after they had produced >/=1 affected offspring, 2 had phenylketonuria diagnosed as a result of transient postnatal hyperphenylalaninemia in an offspring, and 9 had phenylketonuria diagnosed by prenatal screening. Undiagnosed maternal phenylketonuria in North America and Europe is currently estimated at 1 case/100,000 births; this rate could be higher elsewhere.

CONCLUSIONS

Physicians and midwives should consider a protocol of selective prenatal screening or case finding to detect undiagnosed phenylketonuria among their patients.

Neonatal neurological assessment of offspring in maternal phenylketonuria

This study assesses the impact of prenatal and postnatal factors in maternal phenylketonuria (PKU). The Dubowitz Neurological Assessment of the Preterm and Full-term Newborn Infant was administered within the first 8 days of life to 56 offspring of women with PKU and 45 controls. Follow-up testing of the maternal PKU offspring at age 1 year consisted of the Bayley Scales of Infant Development and the Receptive-Expressive Emergent Language Scale (REEL). In addition, the Home Observation for Measurement of the Environment (HOME Scale) was given. Birth weight was lower (z = 2.0, p = 0.045), birth length was lower (z = 2.1, p = 0.03) and birth head circumference was smaller (z = 3.5, p = 0.0005) in the maternal PKU offspring than in the control infants. Examiners rated 29% of the maternal PKU offspring and 9% of the control infants abnormal (Fisher's exact test, p = 0.01). At 1 year of age, 19% of the maternal PKU offspring attained a Bayley Developmental Quotient (DQ) and a score on the Bayley Motor Scale below 85; 19% had receptive language delay; and 26% had expressive language delay. The gestational age at which the mother attained metabolic control was an important factor associated with birth measurements, the Dubowitz Rating and subsequent developmental scores. The Dubowitz Neurological Assessment score did not predict developmental outcome (chi-square = 1.3, p = 0.53), while the HOME score correlated with the DQ (r = 0.36, p = 0.02). In logistic regression analyses, the home environment was a greater determinant of risk for a low DQ than whether or not the mother attained metabolic control prior to pregnancy (OR = 0.85, p = 0.02). These results suggest that treatment strategies addressing both prenatal and postnatal factors will most effectively reduce risks in maternal PKU.

Intellectual, neurologic, and neuropsychologic outcome in untreated subjects with nonphenylketonuria hyperphenylalaninemia. German Collaborative Study on Phenylketonuria

Based on the serum phenylalanine levels under free diet patients with hyperphenylalaninemia are classified as "classical" (>1200 micromol/L), "mild" (600-1200 micromol/L), or "non-phenylketonuria (PKU)-hyperphenylalaninemia" (<600 micromol/L). Recent studies revealed intellectual, neurologic, and neuropsychologic deficits as well as abnormalities of cerebral white matter (magnetic resonance imaging, MRI) in patients with early and adequately treated PKU. In addition deficits in IQ were reported for a group of 4-y-old patients with untreated mild PKU and non-PKU hyperphenylalaninemia (serum phenylalanine levels below 900 micromol/L). As a consequence, a lifelong diet with serum phenylalanine levels below 400 micromol/L was recommended even for those patients with serum phenylalanine levels remaining consistently between 400 and 600 micromol/L. Generally patients with non-PKU hyperphenylalaninemia were not treated, as a normal outcome was suspected, but the clinical development of patients with non-PKU hyperphenylalaninemia was not so far studied systematically. We assessed 28 untreated patients with non-PKU hyperphenylalaninemia (age: mean = 21.8, SD = 4.2 y) for IQ, school and job career, clinical-neurologic development, fine motor performances, selective and sustained attention, as well as for frontal lobe-dependent "executive functions." In addition, cranial MRI was obtained in 10 of these patients. Compared with healthy control subjects, matched for age, sex, and socioeconomic status, the patients reached normal results in all clinical and psychometric tests. Cranial MRI revealed no abnormalities. Additionally, no significant correlations between serum phenylalanine levels and test results were obtained. In the absence of any demonstrative effect, treatment is unlikely to be of significant effect in patients with non-PKU hyperphenylalaninemia.

Newborn phenylketonuria (PKU) Guthrie (BIA) screening and early hospital discharge

Recent policies of early discharge of postpartum mothers and their infants has raised concerns of possible decreased sensitivity in Guthrie bacterial inhibition assay (BIA) phenylketonuria (PKU) screening resulting in missed cases. In order to assess the potential impact of early discharge from hospital on neonatal screening for PKU and its variants, we performed 18 standard BIA screening tests on 11 newborn infants with the disease. Blood spot samples were collected from 1 to 24 h after birth and were analyzed at the Ontario Ministry of Health newborn screening laboratory according to the routine screening protocol. Except for one 4-hour postnatal sample from an infant with 'non-PKU mild hyperphenylalaninemia' (MHP) all blood samples showed phenylalanine levels > or = 240 mumol/l, irrespective of the age of the baby. During our 29 year experience with neonatal PKU screening (3.9 million infants tested), employing a cutoff blood phenylalanine of 240 mumol/l in blood spots obtained at > or = 24 h of age, only two biological false negative (one confirmed) tests were discovered in infants subsequently shown to have classical PKU: another three false negative tests were discovered in sibs of infants with MHP. The sensitivity of the screening test was 99.2% for infants with classical and mild PKU. Ascertainment of patients with MHP is unknown and is very likely incomplete. Over a 3-year period (1992-4) the specificity of the test was 99.9% for those screened after 24 h. The positive predictive value was 12.8%. Although early discharge may have an impact on other screened diseases, we conclude, from our studies, that early discharge may not affect the detection of infants with classical and mild (atypical) PKU, but would probably increase the number of infants with MHP missed using the BIA and a cutoff level of 240 mumol/l. Because of our experience and that of others, we recommend that neonates be at least 12 h of age before initial BIA PKU screening be carried out. To confirm this recommendation further prospective studies should be initiated.

Maternal hyperphenylalaninaemia as a cause of microcephaly and mental retardation

We attempted to evaluate the role of maternal hyperphenylalaninaemia (HPA) as an isolated cause of mental retardation and microcephaly in children. This transversal study observed the plasma phenylalanine from mothers of 161 children with mental retardation and/or microcephaly of unknown origin. In this sample, we found two women with previously undiagnosed HPA, a frequency (2/161) higher than expected for our general population (1:12 500) (p < 0.001). We concluded that the plasma phenylalanine levels should be determined during preconceptional evaluation of every woman of reproductive age that already has had a child affected either by mental retardation or microcephaly of unknown cause. It is particularly significant where women currently having their pregnancies have not been screened for phenylketonuria as newborns, as happens in most developing countries.

German Maternal Phenylketonuria Study

The German maternal phenylketonuria (MPKU) Study began in 1989 and since 1992 works together with the American-Canadian MPKU Study. Main goals of the study are: (1) to find women with phenylketonuria (PKU) and mild untreated hyperphenylalaninaemia (HPA); (2) to inform them about the risks of an untreated pregnancy with PKU and HPA; (3) to evaluate the efficacy of the phenylalanine (Phe) restricted dietary treatment prior to and during pregnancy by following the physical and cognitive development of offspring from treated pregnancies. An interim report of the study is presented. Until now, 43 pregnancies have been followed. They resulted in 34 live births, 24 from women with PKU and 10 form women with HPA. There are significant negative correlations between the gestational age in which the dietary control (blood Phe level < 360 mumol/l) was reached and pregnancy outcome as measured by growth parameters and early cognitive and motor developmental quotients at the age of 2 years. For minimizing risks of MPKU, preconceptional dietary control is strongly recommended. Tracking and timely information of young women about risks of MPKU is of outmost importance.

Maternal non-phenylketonuric mild hyperphenylalaninemia

Unlike maternal phenylketonuria (PKU) which produces severe birth defects when untreated during pregnancy, maternal non-PKU mild hyperphenylalaninemia (MHP) has a less severe impact but whether it is benign or may have long-term consequences for offspring has been unclear. From an international survey of maternal MHP we obtained information about 86 mothers (blood phenylalanine (Phe) 150-720 mumol/l), their 219 untreated pregnancies and 173 offspring. Spontaneous fetal loss and congenital anomalies were no more frequent than normally expected. Median Z-scores for birth length and birth head circumference and offspring IQ (100), however, were significantly lower for maternal Phe > 400 mumol/l than for maternal Phe < 400 mumol/l, in which the median offspring IQ was 108. Data on maternal MHP from the prospective Maternal PKU Collaborative Study (MPKUCS) are as yet incomplete but seem to be conforming to the general pattern of the international survey. We conclude that maternal blood Phe levels above 400 mumol/l in maternal MHP are associated with lower birth measurements and slightly lower offspring IQ. It would seem that dietary intervention to lower the maternal Phe levels to below 400 mumol/l might be indicated in maternal MHP pregnancies with the higher blood Phe levels.

Fetal ultrasonography in maternal PKU

Maternal phenylketonuria (PKU) is teratogenic and results in birth defects that include microcephaly, mental retardation, congenital heart disease, and intrauterine growth retardation. Treatment with a low phenylalanine diet can prevent or reduce the severity of the complications. Optimal benefit, however, requires frequent monitoring with fetal ultrasonography as a critical element. We have studied ultrasonography in 39 pregnancies enrolled in the Maternal PKU Collaborative Study and followed at our centre. First-trimester examinations in 24 pregnancies resulted in the discovery of non-viability in five. In each, this led to discontinuation of the difficult and expensive diet. Among the 33 pregnancies with second-trimester evaluation, congenital heart disease was identified in five. Two of these pregnancies were terminated. Microcephaly as determined by biparietal diameter (BPD) was identified in the second trimester in only one of nine fetuses who had microcephaly at birth. Among 20 pregnancies with third-trimester ultrasound, fetal microcephaly was identified by BPD in three of seven who had birth microcephaly. We conclude that fetal ultrasonography in maternal PKU is valuable during the first trimester in identifying non-viable pregnancies and determining gestational age and is also valuable during the second trimester in identifying congenital heart disease and perhaps other major anomalies, but not in identifying fetal microcephaly. Third-trimester ultrasound seems to be of limited usefulness.

[Pregnancy and the child of a mother with phenylketonuria]

Pregnant women with hyperphenylalaninemia are at high risk of spontaneous abortion and of giving birth to infants with congenital malformations, microcephaly and mental defect. Among mothers whose phenylalaninemia is greater than 1200 mumol/L (20 mg/100 mL), 95% have at least one child with mental retardation. A low phenylalanine diet with a good control of phenylalaninemia, started before conception, reduces this risk, better results being obtained when plasma phenylalanine levels are maintained below 360 mumol/L (6 mg/100 mL) as compared with levels maintained between 360 to 600 mumol/L (6-10 mg/100 mL). Thus, systematic contraception and planned pregnancies must be recommended in all hyperphenylalanemic young women. This implies early information of phenylketonuric teenage girls and their parents. In addition, efforts must be made to join and inform all women having had hyperphenylalaninemia at birth, whether they received a dietary treatment or not. It is also important that general practitioners, pediatricians and obstetricians be aware of the high recurrence risk in hyperphenylalanemic women who gave birth to a microcephalic or malformed infant.

Maternal phenylketonuria: a metabolic teratogen

The maternal phenylketonuria (PKU) syndrome refers to the teratogenic effects of PKU during pregnancy. These effects include mental retardation, microcephaly, congenital heart disease, and intrauterine growth retardation. In untreated pregnancies wherein the mother has classic PKU with a blood phenylalanine level > or = 1,200 microM (20 mg/dl), the frequencies of these abnormalities in offspring are exceedingly high, approaching 75-90% for microcephaly and mental retardation and 15% for congenital heart disease. There is a dose response relationship with progressively lower frequencies of these abnormalities at lower phenylalanine levels, both in the pregnancies of women with variants of PKU and in treated classic PKU pregnancies. The pathogenesis of this syndrome is unknown; it may be related to inhibition by phenylalanine of large neutral amino acid transport across the placenta or to direct toxicity of phenylalanine and/or a phenylalanine metabolite in certain fetal organs. A mouse model for PKU now exists, and studies of maternal PKU in this model are in progress. The treatment of maternal PKU consists of biochemical control through a phenylalanine restricted diet during pregnancy. The best results are obtained with diet initiation before conception or no later than the earliest weeks of pregnancy. Women with PKU and their families require much psychosocial support to meet the strict requirements of a maternal PKU pregnancy, including compliance with a difficult diet. With such compliance, however, it seems that bearing normal or near normal offspring is possible.