The maximum chances of HIV infection being transmitted to the fetus/ infant in an HIV infected mother are during:

Veical transmission of HIV (out of total 100 %) before 36 weeks: 20 % days before delivery: 50 % intrapaum: 30 % The risk of transmission during breast feeding may be as high as 30-40%. Also note that at all stages, risk of HIV transmission is directly related to maternal HIV viral load. Therefore maximum risk , out of given options is peripaum (days before delivery + intrapaum + immediate post paum) So to bring this infection rate less than 2 % all the HIV positive pregnant woman are staed on anti-retroviral (A) drugs irrespective of CD 4 count throughout pregnancy.