A study of hundreds of women, carried out at Imperial College London, found that subtle changes to the bacteria present in the vagina were strongly associated with the mother's waters breaking early and preterm birth – the baby being born before 37 weeks. The study findings were published in the journal BMC Medicine

However, when this occurs before 37 weeks, termed premature rupture of membrane (PPROM), the baby is likely to be born prematurely. After the membranes rupture, the baby remains without the protective membrane and is at increased risk of infection. In order to reduce this risk, the women whose waters have broken early are given intravenous antibiotics as standard practice on the NHS.

Researchers looked at the impact of premature rupturing of the membrane and antibiotic treatment on the vaginal microbiota, taking swabs from the vaginas of pregnant women at different points during their pregnancy and analysing them to reveal the types of bacteria present, their proportions and any changes.

Samples were collected from a prospective group of 250 pregnant women with and without risk factors for giving birth prematurely of which 27 did in fact have a premature birth. They also collected samples from a second, smaller group of 87 women who presented to hospital with premature membrane rupture

Analysis of the team's samples revealed that premature membrane rupture was associated with a shift in microbiota, with a drop in Lactobacillus and an increase in other types of bacteria, including potentially harmful bugs such as Staphylococcus and Streptococcus.

The team also analyzed samples from the small group of women with premature rupture before and after the preventative antibiotic treatment – oral erythromycin, four times a day for 10 days. Swabs were taken before treatment and then at 48 hours, one week and two weeks.

For those women whose microbial makeup was dominated by Lactobacillus before the treatment, the antibiotics resulted in a decline in Lactobacillus and a greater diversity of bugs. However, in those women with reduced Lactobacillus to begin with, the treatment was beneficial in some, reducing the amount of potentially harmful bacteria as well.

The study also revealed associations between specific vaginal bacteria and newborns who developed sepsis following delivery. While the mothers of healthy babies were dominated by Lactobacillus, samples from the mothers of newborns with sepsis revealed a greater diversity of bacteria, including the presence of Streptococcus and E.coli.

The group adds that further studies are needed to assess current clinical guidelines for women with premature rupture and that developing alternative ways to treat women, such as using more selective antibiotics, could potentially improve outcomes for women and their babies.

Dr Richard Brown said, "The aim of antibiotic treatment in PPROM is to reduce the risk of ascending infection. Our results suggest that a more personalized approach targeting only those women likely to benefit from antibiotics may prove more beneficial than the current 'one treatment fits all' approach."