What physiological changes in pregnancy contribute to an increased incidence of urinary tract infections?

The correct answer highlights the physiological changes during pregnancy that can lead to an increased risk of urinary tract infections (UTIs). In pregnancy, hormonal changes cause relaxation of smooth muscles throughout the body, including those in the urinary tract. Specifically, the relaxation of the smooth muscle of the urinary sphincter can compromise its ability to maintain closure, potentially leading to urine retention, which is a risk factor for the development of UTIs.

This relaxation reduces the effectiveness of the sphincter mechanism that normally helps to prevent the backflow of urine and the entry of bacteria into the bladder and urinary tract. The relaxation effect is particularly pronounced due to elevated levels of progesterone.

While other factors such as relaxation of the bladder's smooth muscle and inadequate emptying of the bladder may also contribute to urinary retention and increased susceptibility to infections, the direct role of the urinary sphincter’s relaxation is particularly significant in the context of preventing infections. Additionally, an increased incidence of bacteriuria also ties into this concern as it is a risk factor that may arise from underlying issues such as those caused by sphincter and bladder changes, but the mechanical and hormonal alterations directly influencing urinary flow and retention are critical in understanding the origin of the increased UTI incidence during pregnancy