Passage of Potentially Noxious Substances into the Fetus and Infant

2 PASSAGE OF POTENTIALLY NOXIOUS SUBSTANCES INTO THE FETUS AND INFANT 

The placenta is permeable to many types of drugs and chemical pollution. Especially lipophilic and nonionized substances with low molecular weight ( 500-1000) can pass rapidly. Not all fetal blood passes the liver. Furthermore, the liver develops its metabolizing capacity only gradually, which is far from complete at birth. Renal excretion of adverse chemicals is likewise very limited during most of the gestation period. Therefore a wide range of potentially harmful chemicals which easily reaches the fetus may hardly or not 

After birth the infant can be affected by toxic substances taken up by the lactating mother and excreted into the milk. Different drugs and environmental polluting chemicals have been shown to be excreted in milk. Clinically prescribed drugs normally do not reach high levels in breast milk, but there are exceptions. Especially chemicals which cannot be metabolized by the infant may occur in high levels, and thus become potentially harmful (refs. 64, 65). 

The blood-brain barrier in the fetus and newborn infant is highly permeable for chemicals, since it develops gradually a permeability comparable to that of the adult only by the first year of life (ref. 3). 

Alcohol ingested by the pregnant woman reaches comparable levels in mother and fetus, but may disappear from the fetus at a much lover rate than from the mother because of accumulation in the amniotic fluid (refs. 60, 69, 70). Hutchings (ref. 49) found indications that prenatally administered methadone accumulated and persisted in neonatal rat brain and liver for long periods, while the half life of this substance in adult rat plasma was only a few hours. 

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