An 18-year-old pregnant woman presents with nocturia and increased urinary frequency. She is at 18 weeks gestation and she has no past medical history. She has no suprapubic fullness; her fundal height is appropriate for her weeks of gestation.
Diagnostic findings are as below:
UrineNo cells, no blood, no protein; trace glucose
specific gravity 1.020
Fingerstick glucose85 mg/dL
Renal ultrasound Bilateral mild hydronephrosis, no noted ureteral abnormalities; bladder not distended post-void
Serum creatinine0.5 mg/dL

What is the most likely explanation for her urinary frequency and nocturia?
a. Physiological changes in pregnancy leading to increased renal blood flow and glomerular filtration rate
b. Urinary tract infection
c. Pregnancy-induced hormonal changes leading to increased bladder sensitivity
d. Compression of the bladder by the growing uterus