When a drug leading to selection of a resistant viral population is withdrawn the original wildtype virus, over time, becomes the predominant virus again. However, low frequency (below level of detection) of drug resistant viral sub-populations may be retained which could mean that the drug resistant variant might reappear rapidly should the same drug be reintroduced as part of combination therapy at a later stage.Therefore this study is proposed to evaluate whether there is an impact of single dose nevirapine for the prevention of MTCT on the subsequent response to a standard HAART regimen that contains nevirapine.