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Objective: To study the effect of elective Cesarean section and zidovudine prophylaxis on vertical HIV transmission., Design: Prospective study., Setting: Obstetric and paediatric clinics in Switzerland., Participants: Children of mothers with HIV infection identified before or at delivery., Interventions: Routine use of elective Cesarean section for HIV-infected parturients by some Swiss centres since 1985. National recommendation for zidovudine prophylaxis in mid-1994., Main Outcome Measure: HIV infection status of children., Results: In a cohort of 494 children born at least 6 months before the analysis date, 67 out of 414 children with known infection status were found to be infected, giving an overall transmission rate of 16.2% [95% confidence interval (CI), 13.0-18.51. Elective Cesarean section with intact membranes and without previous labour was associated with a lower transmission rate of 6% [odds ratio (OR), 0.29; 95% CI, 0.12-0.70; P = 0.006 versus other delivery modes]. Transmission rate was intermediate after spontaneous delivery or non-elective Cesarean section (18%), and higher after obstetric interventions (27%; test for trend, P < 0.001). Since mid-1994, 78% of all women with registered pregnancies have received some form of zidovudine prophylaxis. Transmission rate was reduced from 17 to 7% after any zidovudine exposure (OR, 0.4; 95% CI, 0.11-1.41). Combined use of elective Cesarean section and zidovudine resulted in a 0% transmission rate (none out of 31), compared with 8% (seven out of 86) after elective Cesarean section without zidovudine, 17% (four out of 24) after zidovudine alone, and 20% (55 out of 271) after no intervention., Conclusions: Elective Cesarean section and zidovudine prophylaxis appear to have an additive effect in the prevention of vertical HIV transmission.