Firstborn twin runs higher AIDS risk.

The twin that braves the birth canal first faces the greater risk of infection with mother's AIDS-causing human immunodeficiency virus (HIV), a new study indicates.

A decade after the first AIDS cases surfaced, scientists still don't completely understand how the deadly virus passes from mother to child. Some studies have hinted that mothers can pass HIV to an embryo or fetus in the womb or during delivery. Others have suggested that newborns can catch HIV from infected breast milk (SN: 8/31/91, p.135). The new study supports the theory that some babies contract HIV infection during passage from the womb.

AIDS researcher James J. Goedert of the National Cancer Institute's Viral Epidemiology Section and his colleagues obtained data on 66 sets of mostly bottle-fed twins born to HIV-infected mothers in nine countries. Goedert's team collected information on birth order, delivery method and whether or not the infants showed signs of HIV infection. Babies were at least 6 months old at the time of data collection.

Among the firstborn twins, 50 percent of those delivered vaginally and 38 percent of those delivered by cesarean section had HIV infection, the researchers report in the Dec. 14 Lancet. The twin who lagged behind had a better chance of escaping the viral threat: With either delivery method, 19 percent of these babies had HIV infection.

Goedert says these findings suggest that vaginal delivery can put infants at high risk of contracting their mother's HIV infection. He speculates that the trip through the cervix and vagina may expose a firstborn twin to large amounts of bloody, HIV-laced secretions. By the time the second twin gets to the birth canal, he adds, the sibling has cleared out much of the bloody fluid -- and perhaps the HIV as well.

Goedert suspects that cesarean sections are risky for firstborn twins when doctors wait too long after the protective amniotic sac breaks and the baby nearest the cervix is exposed to an influx of maternal HIV. As in vaginal births, the first twin delivered in a cesarean section is the one closest to the cervix.

The researchers say their data also hint that genetics may play a role in a baby's vulnerability to HIV infection.

The new findings await confirmation in laboratory studies and larger clinical trials, notes Howard L. Minkoff, an AIDS researcher at the State University of New York Health Science Center at Brooklyn. Furthermore, scientists still don't know whether solo babies, who also endure traumatic, bloody deliveries, run the same HIV risk as firstborn twins. "It's early data," Minkoff says.

However, notes Goedert, if further studies clearly establish a delivery-related HIV threat, then scientists can begin working on methods of preventing mother-to-baby HIV transmission, potentially saving thousands of infants worldwide.