Fetal ultrasound showed enlarged renal pelvis and large head. So could it be a syndrome?

May or may not. Enlarged renal pelvises are common urinary tract anomalies. A large head can be familial. So not likely. But then there is much ultrasound can't diagnose. Just have to wait and see.
Usually not. These findings are frequently seen with maternal diabetes mellitus and are not immediately suggestive of any other syndrome. Having said that, prenatal ultrasound cannot definitively diagnose most fetal syndromes and can miss a lot of anomalies.