Factors in fetal drug exposure
Factors determining the effects of teratogens
Factors that determine the effects of teratogens include the following:
- Dose reaching fetus
Most drugs cross the placenta by simple diffusion. Many actually reach 50%–100% of the concentration in fetal blood as that in maternal blood. The rate at which drug transfer takes place depends on the drug characteristics, the blood flow of the uterus, and maternal kinetics.
- Point in development when drug exposure occurs
During the period from conception to 2 weeks, there is a relative resistance to drug effects. Usually exposure during this time produces an “all or none” effect. The remainder of the first trimester (week 4 - 10) is the most critical time for organ malformation. Unfortunately, this is also a time when many women are unaware of their pregnancy. Drugs that reach the embryo at this point may produce abortion, no effect at all, an anatomic defect (teratogenesis), or a subtle metabolic or functional defect that may not be detected until later in life. During the second and third trimester, known as fetogenesis, drugs are less likely to be associated with major malformations, but they may influence neurologic development, growth, physiologic and biochemical functioning, mental development, and reproduction.
- Duration of exposure
- Environmental factors
- Susceptibility (resistance) of the fetus