Pregnant women urinate more frequently due to hormonal changes that cause increased blood flow through the kidneys, explains Baby Center. This causes the kidneys to work more efficiently, leading to increased urine production. The uterus also exerts pressure on the bladder, reducing its urine storage capacity and making the pregnant woman urinate more frequently.
Increased blood flow to the kidneys causes them to filter more urine and deliver it to the bladder faster. This is beneficial as the kidneys are filtering and expelling waste for two or more, which is another reason the pregnant woman urinates more frequently. Over the course of the pregnancy, a woman's blood volume increases by up to 50 percent, according to Baby Center. Therefore, her kidneys process more fluid, and some of it ends up in the bladder.
Trips to the bathroom are more frequent at night. This is because as the woman lies down, and the fluid that had been retained in her legs and feet during the day makes its way back into the bloodstream and ends up in the bladder, explains Baby Center.
During the first trimester, the uterus grows bigger and presses on the bladder, states Parents. In the third trimester, the baby drops downward and exerts pressure on the bladder, causing frequent bathroom breaks.