Two protocols for prophylaxis by antibiotics in labour were compared to see how effective they were in preventing materno-fetal infection in patients who had signs of chorio-amnionitis. There was in increase in E. coli infections in 40% of liquor which were resistant to amoxycillin which had been used right up to now. 135 cases of patients who had received antibiotic treatment in labour were studied. 61 of these were treated with amoxycillin and 74 with piperacillin. The results show that there was a drop of 11.05% in the number of newborns who were infected in the group of patients who were treated with piperacillin. It was not possible using piperacillin to avoid all infections of the mother and fetus. The severity of the infection in the fetus, as judged by the Apgar score and the opinion of the paediatricians, was less serious in the group that was treated with piperacillin. The death rate for both groups was identical: one in each group. Both were caused by E. coli infections. A drop of 11% in post partum maternal infections was noted. Its was shown that piperacillin was more efficacious, but this way of preventing of infections in the mother and fetus lay in finding as early as possible the situation that could increase the risk of infection.