Bacteria that cause infection in the lower genital tract (infection of the cervix or vaginal wall) can travel upwards through the cervix and infect the fetal membranes. This can weaken the membranes enough to allow them to rupture.

Evidence of infection in a woman with PROM

Fever: the woman may complain of feeling feverish, or you may record her temperature of 38°C or more.
The vaginal discharge may have an offensive smell and the colour may be changed from watery to cloudy.
She may have an increased pulse rate (more than 100 beats/minute).
The fetal heart beat may increase to 160 beats/minute or more.
She may feel pain in the lower abdomen, particularly when it is touched.