What causes urinary frequency in pregnancy?

During pregnancy, the kidneys will enlarge and the ureters dilate. In the second half of your menstrual cycle, starting at ovulation, your body produces progesterone. Progesterone production will continue if pregnancy occurs or decrease if not pregnant, triggering menstruation. Progesterone helps prepare the lining of the uterus for implantation and also has a relaxing effect on smooth muscles.

Progesterone's relaxing effect is believed to play a role in causing the ureters to dilate as well as relaxing the bladder. This combined with the increasing pressure on the bladder for the expanding uterus can cause urinary frequency.

What else could cause urinary frequency?

Urinary frequency is a symptom of diabetes mellitus. A urinary tract infection could also be responsible for urinary frequency.

Also see:
Implantation Bleeding
Dizziness, Symptom of Pregnancy
Am I Pregnant? How Early Can I Test?