Why am i urinating often during my first trimester. ?

More blood to kidney: During pregnancy, even as early as the first month, your blood vessels dilate and blood volume increases. This causes more blood flow to your kidneys, which creates more urine, so you'll need to go more often. As your uterus grows, it will press on your bladder also. If you are experiencing any burning or pain during urination, see your doctor as you may have a urinary tract infection.

Enlarging uterus: You are urinating more frequently because both your bladder and uterus are pelvic organs. The enlarging uterus both occupies some of the space normally occupied by the bladder as well as exerting pressure on the bladder. Thus your bladder feels full sooner than before pregnancy. Total urine output depends upon your fluids intake, thus your output will not have changed unless you are drinking more.