Why do some metabolic disorders cause symptoms only after birth?

Some metabolic disorders, specifically inborn errors of metabolism, do not manifest symptoms until after birth because of the role of the placenta. During pregnancy, the mother’s own metabolism can compensate for the infant's impaired metabolic processes by processing substances that the infant cannot handle on their own.1

This compensatory mechanism is particularly common in disorders involving relatively small molecules, such as amino acids, simple sugars, and certain hormones. Because the mother’s system effectively manages these metabolites through the placental exchange, the fetus remains unaffected while in the uterus. However, once the infant is born and separated from the mother’s circulatory system, they must rely entirely on their own metabolic pathways. If a functional enzyme is absent or defective, the infant can no longer process these substances, leading to an accumulation of harmful chemicals and the subsequent onset of symptoms.1234