A condition characterized by mental retardation in which the fetal brain is damaged by phenylalanine in utero as a result of untreated or uncontrolled pre-pregnancy phenylketonuria (PKU) is known as maternally derived phenylketonuria (MDP). Phenylketonuria is an autosomal recessive metabolic disease, the causative gene was cloned in 1986, and there are more than 400 mutations. Excessive amounts of phenylalanine are produced in pregnant patients. Phenylalanine is a teratogenic substance. Phenylalanine in the mother’s body can pass through the placenta to produce a toxin effect on the development of the fetus, especially on the nervous system damage, is abnormal fetal development. Clinical features: Mental retardation, microcephaly and congenital heart disease are the main clinical manifestations of maternal phenylketonuria. Fetal growth is usually impaired intrauterine. Patients have central nervous system abnormalities and mental retardation with an average IQ of 40. microcephaly and congenital heart disease are common. Other congenital anomalies include cleft lip/palate and pyloric stenosis. Treatment: There is no specific treatment for maternal PKU, which is mainly symptomatic for mental retardation. Surgery may be considered for those with organic congenital defects. Once the fetus is damaged, it is difficult to recover and reverse the pathologic changes in the central nervous system after birth, and congenital malformations can only be lifelong changes. Prevention: 1. Measurement of maternal blood phenylalanine concentrations can predict the risk of fetal disease. 2.Genetic counseling services are given before conception to educate patients about the prevention of mother-derived PKU. 3, It is advocated that pregnant women should have PKU under control three and a half months or more before planned conception. 4. Once pregnancy is detected, the phenylalanine diet should be strictly restricted and the maternal phenylalanine blood concentration should be controlled within the normal range. 5, newborn screening for PKU and its follow-up is absolutely necessary; special attention should be paid to family care, including diet, after the birth of the child, to avoid mild or undiagnosed children continue to suffer damage.