Final answer:
The major risk factor for postpartum infection is a Cesarean birth, due to it being a major abdominal surgery which increases the risk of post-surgical complications like infections.
Step-by-step explanation:
The major risk factor for a postpartum infection is Cesarean birth (Option A). Because it is a major abdominal surgery, a Cesarean section can lead to post-surgical complications, which includes an increased risk of infection. The other options such as primiparity (having your first child), breastfeeding, and being over the age of 35 may have other associated risks, but they are not the major risk factors for infection postpartum.
It is during the period of puerperium, when the reproductive system is returning to a nonpregnant state, that the mother is particularly susceptible to infections. Nosocomial infections, such as puerperal sepsis, may originate from the genital tract, urinary tract, breasts, or a surgical wound like that from a Cesarean section. Adherence to handwashing protocols and use of antiseptics can greatly reduce the incidence of puerperal fever.