Cesarean + AZT = almost no HIV transmission.

Pregnant women with HIV, the virus that causes AIDS, can in nearly all cases prevent their babies from contracting the disease during birth. These mothers must take the drug AZT and deliver by planned cesarean section, two studies show.

French scientists report that of 133 HIV-positive women who took AZT and then gave birth by elective cesarean before contractions started, only one transmitted HIV to an infant, a rate of less than I percent. The researchers had followed the progress of 2,834 HIV-positive mothers between 1985 and 1996.

They found that babies born vaginally to 625 mothers taking AZT in this group contracted HIV 6.6 percent of the time. Emergency cesarean sections that were required by complications after contractions started in 114 other women taking AZT yielded an HIV transmission rate of 11.4 percent.

Of 1,877 HIV-positive mothers not treated with AZT, 17.2 percent transmitted HIV to their infants. The method of birth didn't affect their risk, the researchers report in the July 1 Journal of the American Medical Association and at the 12th World AIDS Conference this week in Geneva.

The results match those of Swiss researchers who have found in an ongoing study that all 42 children born by planned cesarean to HIV-positive mothers being treated with AZT tested negative for HIV. The Swiss findings, first described in the Jan. 22 AIDS, were updated this month.

The report documented HIV infections in 20.3 percent of 271 infants born vaginally to mothers who were not receiving AZT. Of infants born by cesarean section to mothers in this group, 16.7 percent (4 of 24) were HIV-positive. Mothers receiving AZT passed the virus along in 8.1 percent (7 of 86) of the infants born vaginally.

During vaginal childbirth, the infant can swallow blood or other fluids from the mother, or these fluids can come into contact with the child's mucous membranes or abrasions on the baby's skin, says Swiss study coauthor Christian Kind, a neonatologist at Kantonsspital in St. Gallen, Switzerland.

Opportunities for infection can arise before delivery as well. Labor contractions place the child at risk because the placenta may leak maternal blood that can carry viruses to the baby. Also, if the amniotic sac surrounding the fetus ruptures at least 4 hours before delivery, the child can become exposed to HIV, Kind says. Thus, having the cesarean section before contractions start "is absolutely essential," he concludes.

Although the research is valid, any public health approach that includes mass cesareans "has a lot of practical limitations," says Ruth E. Dickover, a virologist at the University of California at Los Angeles School of Medicine. Many women who come to the UCLA clinics arrive late in their pregnancies, have no health insurance to pay for a cesarean delivery, don't have an accurate due date, and don't know their HIV status, she says. For women in the developing world, prenatal care is often nonexistent, and the possibility of widespread cesareans is remote, she notes.