Which factor primarily contributes to urinary frequency in the first trimester of pregnancy?

During the first trimester of pregnancy, the primary contributor to urinary frequency is bladder pressure from the enlarged uterus. As the uterus grows, it exerts pressure on the bladder, reducing its capacity to hold urine and leading to increased frequency of urination. This is particularly noticeable in early pregnancy when the uterus begins to expand and can disrupt the normal function of the bladder.

While other factors, such as hormonal changes and fluid retention, play a role in urinary patterns during pregnancy, they are not as significant in the first trimester as the physical pressure exerted by the enlarging uterus. The hormones, including estrogen, do have effects on the renal system and can influence other aspects of the body's fluid balance, but the key factor in early pregnancy urinary frequency is primarily due to the anatomical changes occurring in the pelvic region.