A 28-year-old primigravida from rural Maharashtra presents for her first antenatal visit at 12 weeks of gestation. She reports irregular menstrual cycles before conception. On examination, her blood pressure is 110/70 mmHg, and abdominal examination is unremarkable. Urine dipstick shows 1+ proteinuria. Serum creatinine is 0.9 mg/dL. Her booking blood tests reveal haemoglobin 10.2 g/dL, blood group O positive, and negative VDRL. Ultrasound confirms a viable singleton pregnancy with normal nuchal translucency. Which investigation should be prioritized at this visit to establish baseline renal function and assess for underlying chronic kidney disease?