A 28-year-old woman with known classical phenylketonuria (PKU) on a phenylalanine-restricted diet since infancy presents to the antenatal clinic at 8 weeks of gestation. Her current serum phenylalanine is 18 mg/dL (normal <2 mg/dL, therapeutic target 2–8 mg/dL). She has normal intelligence and no neurological sequelae. What is the most appropriate immediate next step in management?

Explanation

## Maternal PKU and Fetal Teratogenesis This case illustrates **maternal PKU syndrome** — a critical obstetric complication where maternal hyperphenylalaninemia (not fetal genotype) causes severe fetal damage through placental transfer of phenylalanine and its metabolites. ### Pathophysiology of Maternal PKU Syndrome **Key Point:** The fetus is **not** PKU-affected (it has normal PAH enzyme activity from the normal allele inherited from the father). However, the **maternal** elevated phenylalanine crosses the placenta freely via neutral amino acid transporters, exposing fetal tissues to teratogenic concentrations. **High-Yield:** Maternal PKU syndrome causes: - **Microcephaly** (most common) - **Intellectual disability** (even if fetus is genetically unaffected) - **Congenital heart defects** (VSD, PDA, tetralogy of Fallot) - **Growth restriction** - **Cleft palate** - **Hypoplastic left heart syndrome** Risk correlates with **maternal serum phenylalanine levels**, especially during the first trimester (critical period for organogenesis). ### Management Strategy ```mermaid flowchart TD A[PKU woman planning pregnancy or pregnant]:::outcome --> B{Maternal Phe level?}:::decision B -->|< 2 mg/dL| C[Continue diet, monitor monthly]:::action B -->|2-8 mg/dL| D[Optimize diet, target < 2 mg/dL]:::action B -->|> 8 mg/dL| E[Intensify Phe restriction URGENTLY]:::urgent D --> F[Recheck Phe in 1-2 weeks]:::action E --> F F --> G[Achieve target < 2 mg/dL before organogenesis]:::action G --> H[Monthly monitoring throughout pregnancy]:::action H --> I[Neonatal screening of baby for PKU]:::action ``` **Clinical Pearl:** The **critical window** is the first trimester, when organogenesis is most active. Maternal phenylalanine levels >12 mg/dL carry a 95% risk of congenital abnormalities; levels 2–8 mg/dL carry minimal risk. At 8 weeks of gestation (end of organogenesis), this patient's serum phenylalanine of 18 mg/dL is **above therapeutic target** and poses significant fetal risk. Intensification of dietary restriction is urgent to minimize further damage and protect the remainder of pregnancy. **Mnemonic — Maternal PKU Management:** **PRECONCEPTION-TIGHT-CONTROL** - **P**reconception counselling and optimization - **T**ight control (target <2 mg/dL) throughout pregnancy - **C**ontinuous monitoring (monthly) [cite:Harrison 21e Ch 402]