Followup ultrasound. Prenatal ultrasounds are pretty routine now to verify dates/fetal growth/placental location, etc. One common finding is some studies is a bit of extra space (dilated collection system) within kidney scan that may or may not represent a problem for baby. Babies don't make much pee in the womb but after 2 wks of age a followup study will tell me if it is a real issue. Ask your OB if its another issue.