3-phosphoglycerate-dehydrogenase (3-PGDH) deficiency is an L-serine biosyn thesis disorder, characterised by congenital microcephaly, severe psychomotor re tardation, and intractable seizures. We report prenatal diagnosis of an affected fetus by DNA mutation analysis. Ultrasound assessment showed a reduction in fet al head circumference from the 75th percentile at 20 weeks’gestation to the 29t h percentile at 26 weeks. L-serine was then given to the mother, which resulted in an enlarged fetal head circumference to the 76th percentile at 31 weeks. At birth, the girl’s head circumference was normal, and at 48 months’follow-up, her psychomotor development has been unremarkable. 3-PGDH deficiency is an inbo rn metabolic error that can be successfully treated antenatally. 3-phosphoglycerate-dehydrogenase (3-PGDH) deficiency is an L-serine biosyndrome disorder, characterised by congenital microcephaly, severe psychomotor re tardation, and intractable seizures. We report prenatal diagnosis of an affected fetus by DNA mutation analysis. Ultrasound criterion showed a reduction in fet al head circumference from the 75th percentile at 20 weeks’gestation to the 29th percentile at 26 weeks. L-serine was then given to the mother, which resulted in an enlarged fetal head circumference to the 76th percentile at 31 weeks . At birth, the girl’s head circumference was normal, and at 48 months’ follow-up, her psychomotor development has been unremarkable. 3-PGDH deficiency is an inbo rn metabolic error that can be successfully treated antenatally.