Objectives: To estimate when mother-to-child transmission occurs and i
nvestigate the possible role of maternal factors. Design: We studied v
irological data obtained in the first 3 months of life of 95 infected
newborns born to HIV-1-seropositive mothers included in the French Pro
spective Cohort Study who did not breast-feed. Methods: Comparative We
stern blot analysis of sequential blood specimens from neonates and mo
thers with incomplete antibody patterns enabled us to detect antibody
production In some infants. The results of viral investigation of neon
ate specimens enabled us to describe the acute phase of infection in n
ewborns. Because the process between infection and antibody production
is irreversible, we chose a Markov modelling technique, which is well
suited for staged clinical processes. Results: About two-thirds (65%)
of the infants were considered to have been contaminated during deliv
ery. In the remaining infants, the contamination was estimated to have
occurred in utero and 95% of them had been infected less than 59 days
before delivery. The association between the mother's immunological a
nd virological status and the time of transmission was examined. The g
reater the degree of maternal immunodeficiency at delivery (in terms o
f p24 antigen and Western blot pattern) the higher the risk of in uter
o transmission, showing that vertical transmission is dependent on the
mother's immunological status. Conclusions: These estimates should be
considered when designing strategies to prevent mother-to-child trans
mission.