Paediatric HIV infection is still the most important pandemic, despite the substantial reductions of mother to child transmission achieved in North America and Europe. The total number of people living with the human immunodeficiency virus (HIV) rose in 2004 to reach its highest level ever: an estimated 39.4 million people are living with the virus. This number has been rising in every region, compared with two years ago, with the steepest increases occurring in East Asia, in Eastern Europe and central Asia. Sub-Saharan Africa remains by far the worst-affected region, with 25,4 million people living with HIV at the end of 2004. The AIDS epidemic is affecting women and girls in increasing number in Africa; them make up almost 57% of all people infected with HIV, but became a striking 76% in Sub-Saharan area. This review will focus on the current knowledge available regarding the timing of HIV transmission and the subsequent implications for its prevention. Mother to child transmission can take place during pregnancy, labour, delivery and post-partum, through breastfeeding. Different factors may influence HIV transmission during each of these time periods, and hence interventions to reduce transmission during each of these periods may also require different preventive strategies. The risk of mother to child transmission of HIV infection can be substantially reduced from 15-20% without interventions to less than 2% with the use of antiretroviral therapy during pregnancy, during labour and in the neonatal period, with an elective caesarean section delivery and refraining from breastfeeding. Factors associated with an increased risk of perinatal HIV transmission include advanced maternal blood, prolonged duration of ruptured membranes, and increased quantity of HIV in maternal blood at delivery.