Puerperal sepsis is primarily caused by infections of the uterine lining, known as endometritis. After delivery, the uterine cavity is exposed to bacterial contamination, especially following prolonged labor, premature rupture of membranes, or invasive procedures. The damaged endometrial tissue provides an ideal environment for bacterial proliferation leading to infection. This makes endometritis the most common source of puerperal sepsis, whereas infections like mastitis, urinary tract infections, or perineal tears, although important, are less frequent primary causes of systemic puerperal infection. Prompt recognition and antibiotic treatment targeting uterine infection are essential to reduce morbidity and prevent complications such as sepsis or abscess formation.

Reference: Williams Obstetrics, Cunningham F.G., 25th Edition.