On the basis of these findings, the lower ascertainment of infection status among the US women before entry may reflect poorer access to or uptake of antenatal HIV testing in prior pregnancies or of HIV testing outside antenatal care, or alternatively, the possibility that the women from the USA acquired their HIV infection more recently (i.e. since a previous pregnancy), which is consistent with their younger age compared with the women from European sites, and their somewhat higher CD4 count at entry. This evidence concerns the gene CD4 and infection.