A variety of agents (Table 7.1; p. 152) are known to produce congenital malformations in approximately 2 to 3% of all live-born infants. These agents include viruses, such as rubella and cytomegalovirus; radiation; drugs, such as thalidomide, aminopterin, anticonvulsants, antipsychotics, and antianxiety compounds; social drugs, such as PCP, cigarettes, and alcohol; hormones, such as diethylstilbestrol; and maternal diabetes. Effects of teratogens depend on the maternal and fetal genotype, the stage of development when exposure occurs, and the dose and duration of exposure of the agent. Most major malformations are produced during the period of embryogenesis (teratogenic period; third to eighth weeks), but in stages before and after this time, the fetus is also susceptible, so that no period of gestation is completely free of risk. Prevention of many birth defects is possible, but it depends on beginning preventative measures before conception and increasing physicians' and women's awareness of the risks.
A variety of techniques are available to assess the growth and developmental status of the fetus. Ultrasound can accurately determine fetal age and growth parameters and detect many malformations. Maternal serum screening for alpha-fetoprotein can indicate the presence of a neural tube defect or r other abnormalities. Amniocentesis is a procedure in which a needle is placed into the amniotic cavity and a fluid sample is withdrawn. This fluid can be analyzed biochemically and also provides cells for culture and genetic analysis. Chorionic villus sampling (CVS) involves aspirating a tissue sample directly from the placenta to obtain cells for genetic analysis. Because many of these procedures involve a potential risk to the fetus and mother, they are generally only used for higher risk pregnancies (the exception is ultrasound). These risk factors include advanced maternal age (35 years and older); a history of neural tube defects in the family; previous gestation with a chromosome abnormality; chromosome abnormalities in either parent; and a mother who is a carrier for an X-linked disorder.
Modern medicine has also made the fetus a patient who can receive treatment, such as transfusions, medications for disease, fetal surgery, and gene therapy.
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