A 28-year-old woman from Mumbai attends the antenatal clinic reporting irregular menstrual cycles. She is unsure of her last menstrual period and states it was 'sometime in the last 3 months.' Transabdominal ultrasound is performed for dating. The fetal biometry shows: head circumference 24 cm, femur length 4.2 cm, biparietal diameter 8.5 cm, and abdominal circumference 18 cm. There is no fetal cardiac activity. What is the most likely gestational age and diagnosis?