I. INTRODUCTION- A teratogen is any infectious agent, drug, chemical, or irradiation that alters fetal morphology or fetal function if the fetus is exposed during a critical stage of development.
A. The resistant period (week 1 of development) is the time when the conceptus demonstrates the "all-or-none" phenomenon (i.e., the conceptus will either die as a result of the teratogen or survive unaffected).
B. The maximum susceptibility period (weeks 3-8; embryonic period) is the time when the embryo is most susceptible to teratogens, because all organ morphogenesis occurs at this time.
C. The lowered susceptibility period (weeks 9-38; fetal period) is the time when the fetus has a lowered susceptibility to teratogens, because all organs systems have already formed. The result of teratogen exposure during this period is generally the functional derangement of an organ system.
II. INFECTIOUS AGENTS may be viral or nonviral; however, bacteria appear to be non-teratogenic.
A. Viral infections may reach the fetus via the amniotic fluid following vaginal infection, transplacental^' via the bloodstream after maternal viremia, or by direct contact during passage through an infected birth canal.
1. Rubella virus (German measles) infection during pregnancy can cause the classic triad of fetal cardiac defects: patent ductus arteriosus, pulmonary artery stenosis, and atrioventricular (AV) septal defects. It also causes cataracts and deafness.
2. Cytomegalovirus (CMV), a ubiquitous virus and the most common fetal infection, is the cause of cytomegalic inclusion disease, which affects primarily the central nervous system.
a. CMV results in microcephaly, chorioretinitis, hepatosplenomegaly, cerebral calcification, mental retardation, heart block, and petechiae.
b. The risk of malformations is much higher in infants of mothers who had a primary CMV infection during pregnancy compared with mothers who have had recurrent infections.
3. Herpes virus type 2 (HSV-2) is transmitted by maternal viremia or by direct contact during passage through an infected birth canal.
a. Although herpetic infections are quite common in women, transmission of maternal HSV-2 to the fetus is uncommon (i.e., < 1 in 7500 cases).
b. HSV-2 rarely causes fetal malformations.
C. HSV-2 may result in fetal growth retardation, microcephaly, chorioretinitis, and cerebral calcification
4. Varicella zoster virus (VZV; chickenpox) is transmitted to the fetus, in 24% of the cases, following maternal varicella infection during the last month of pregnancy. VZV results in fetal skin scarring, limb hypoplasia, rudimentary digits, club foot, microcephaly, and mental retardation.
5. Human immunodeficiency virus (HIV) is believed by some to cause acquired immunodeficiency syndrome (AIDS). HIV does not cause any congenital malformation.
1. Toxoplasma gondii is a protozoan parasite that is found particularly in cats. It is transmitted to the fetus transplacentally. T. gojidii may cause miscarriage, perinatal death, chorioretinitis, microcephaly, and cerebral calcification.
2. Treponema pallidum is a spirochete that causes syphilis. It is transmitted to the fetus transplacentally. This spirochete can cause miscarriage, perinatal death, he-patosplenomegaly, joint swelling, skin rash, anemia, jaundice, metaphyseal dystrophy, abnonnal teeth (Hutchinson teeth), and changes in cerebrospinal fluid. Antibiotics given to the affected mother usually provide adequate therapy for the fetus.
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