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is that the virus penetrates the mucosal lining of the mouth or gastrointestinal tract of infants.

Potential variables in vertical transmission have been identified. These include HIV-related factors, such as HIV RNA level; strain variation (genotype); biologic growth characteristics (phenotype); plasma versus genital tract viral load; genotypic resistance; CD4 cell count; and maternal immune response. Maternal and obstetric factors have also been identified: clinical stage, STDs or other coin-fections, vitamin A deficiency, substance abuse, cigarette smoking, antiretroviral agents, sexual behavior, gestational age, duration of membrane rupture, placental disruption-abruption, chorio-amnionitis, invasive fetal monitoring, episiotomy, forceps use, and vaginal versus cesarean delivery. Finally, fetal neonatal factors have been identified, including immune system immaturity and genetic susceptibility.

Pregnancy and HIV Disease

Early findings in pregnant women indicated that those with T4-cell counts of less than 300/^L of blood were more likely to experience HIV-associated illness during pregnancy. Pregnant HIV-infected women exhibit a greater T4-cell count decline during pregnancy than do women without HIV infection. T4-cell counts in the HIV-infected do not return to prepregnancy levels. However, the overall declines in counts of HIV-infected women likely represent declines that would have occurred in the absence of pregnancy and suggest that pregnancy does not accelerate disease progression. Note that most studies to date have not shown significant differences in HIV progression or survival between pregnant women and nonpregnant women with HIV infection. The relationship of common pregnancy complications (e.g., spontaneous abortion, stillbirth, perinatal mortality, infant mortality, intrauterine growth retardation, low birth weight, preeclampsia, gesta-tional diabetes, chorioamnionitis, oligohydram-nios, and fetal malformation) to HIV infection has not been definitively established. Note that concerns have been raised that antiretroviral treatment itself may increase the incidence of some pregnancy complications. Note too that both HIV and pregnancy may affect the natural history, presentation, treatment, or significance of certain infections, including vulvovaginal candidiasis, bacterial vaginosis, genital herpes simplex, human papillomavirus, syphilis, cytomegalovirus, toxoplas-mosis, hepatitis B, and hepatitis C. These infections may be associated with pregnancy complications or perinatal infection.

HIV-Infected Babies one challenge in perinatal transmission is determination of which babies are truly HIV-infected as opposed to just carrying the mother's HIV antibodies (which would produce a false-positive test result). HIV transmission can occur during pregnancy (in utero) as well as at the time of delivery (intrapartum) and through breast milk, as noted. HIV transmission is more likely if the virus can be cultured from the mother's blood, or if she has later-stage HIV disease, or if her T4 counts are low; and is more likely to occur in the firstborn than in the secondborn of twins. Note that a baby automatically acquires the mother's antibodies and may carry them for two or more years. usually by the time the infant is 18 months of age, most of the mother's antibodies are gone. The babies may then begin to show signs of clinical AIDS-related illness. But, even at 18 months, a child cannot be unequivocally diagnosed. The most commonly used HIV antibody test to date is not sufficiently accurate until the child is at least two years old.

The rate of perinatal and breast milk HIV transmission is unknown; however, evidence indicates that more than 90 percent of pediatric AIDS cases result from acquisition of the virus in utero from an HIV-infected mother after the first trimester or during the birth process. Studies have shown that a fetus can become infected as early as the eighth week of gestation.

Reports on the probability of a fetus's becoming HIV-infected when the untreated mother carries the virus vary widely. The most often quoted estimate in the United States is from 30 percent to 50 percent.

Prevention and Treatment

Mother-to-fetus infection can be prevented by preventing pregnancy, but this is possible only in cases when the female is aware of her infection and takes measures to prevent pregnancy (i.e., birth control or tubal ligation). In many cases, preg

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