In contrast, evidence on the use of prophylactic antibiotics in assisted vaginal birth is limited to a small trial of 393 women. The researchers are now calling for health authorities to change the advice they provide.
Giving a single dose of antibiotics to mothers who have a medically assisted birth using forceps or vacuum could prevent nearly half of maternal infections, researchers said on Monday, and global health authorities should change their advice.
The estimate that 7,000 infections women who give birth assisted by forceps or with a suction cup could be prevented in the United Kingdom each year.
One in eight births in England are now assisted, around 85,000 a year.
Currently, infections occur in around 16% of assisted deliveries globally and in around 25 percent of cesarean sections.
This is when forceps or a vacuum device called a ventouse suction cup are used to help deliver the baby. Furthermore, women who have assisted births tend to have longer labors, more vaginal examinations, and more tears and surgical cuts, compared with women who do not have assisted deliveries. "Those who received antibiotics were much less likely to have perineal pain, much less likely to have burst stitches, and they had fewer problems feeding their baby as a effect of that pain".
And for every woman who dies from pregnancy-related infection, another 70 women develop a severe infection that can cause long-term problems.
The women were split into two groups, with the first group given a single dose of intravenous amoxicillin, a type of penicillin, within six hours of giving birth. Overall, a third of the births were ventouse and two-thirds of the babies were delivered by forceps.
Only 11 per cent of the 1619 women who received amoxicillin got an infection, compared to 19 per cent of the 1,606 women in the placebo group.
Importantly, women receiving antibiotic prophylaxis were also much less likely to have confirmed culture-proven sepsis compared to those receiving placebo (56% reduction - 11/1,619; 0.6% vs 25/1,606; 1.5%). One in five of these women go on to develop an infection. "The majority are not serious, life-threatening infections, but they do need to be caught early to make sure they don't progress to those major infections".
For every 100 doses of prophylactic antibiotics, doctors avoided the need to administer 168 doses for post-delivery infections that developed later on after birth.
Adopting a policy of giving all women in this group antibiotics could actually reduce overall antibiotic use by 17 per cent, researchers said.
This study involved 3,420 women in Britain.
Rates of perineal wound infection or breakdown (burst stiches), perineal pain, use of pain relief for perineal pain, and need for additional perineal care were also substantially lower in the group who received antibiotics compared to the placebo group.
Those women given antibiotics were also much less likely to need a visit from a GP, nurse or midwife, or to go to hospital due to the way their wound was healing.
They found the total average NHS costs six weeks after birth was £102.50 in the antibiotic group compared with £155.10 for women the given placebo. Pregnancy-associated infection is a major cause of death and serious illness.
Knight says this will have a huge effect on women, not only in terms of infection rate, but also in terms of reducing rates of perineal pain, burst stitches and problems feeding babies as a outcome of pain.
Consultant obstetrician and spokesperson for the Royal College of Obstetricians and Gynaecologists, Pat O'Brien, referred to the trial as "very interesting and well-conducted".
A single dose of preventive medication after assisted childbirth could halve maternal infections. "And we're actually reducing the amount of antibiotics that would be used".