Final answer:
Increased urination in the postpartum period is primarily caused by diuresis and fluid shifts as the body eliminates excess fluid accumulated during pregnancy. Hormonal changes and the pressure of the uterus on the bladder also contribute to increased urine production.
Step-by-step explanation:
The reason for increased urination in the postpartum period is diuresis and fluid shifts. During pregnancy, the blood volume in a woman's body increases substantially, which comes with the need to manage additional waste from both the mother and the fetus. After childbirth, the body begins to eliminate the excess fluid, leading to an increase in urine production. This postpartum diuresis is a way for the maternal body to return to its pre-pregnancy state and is further supported by the fact that the postpartum hormonal changes also contribute to the increased production and flow of urine.
The downward pressure of the uterus on the bladder during pregnancy and an increase in the total volume of urine due to the processing of both maternal and fetal wastes are also factors that contribute to the frequent urination experienced postpartum. Other considerations like stress urinary incontinence, due to the stretching of pelvic floor muscles during childbirth, could be relevant if symptoms persist after the immediate postpartum period.