I didn't have a scan that early but there are black patches in my daughter's head on her 12 week scan, and less so at 20 weeks the brain looked more fully formed then. The brain does actually have a system of fluid filled chambers right inside it. Also the organs do take on some strange shapes in the earlier weeks before growing into their final shape, they don't start out as a tiny version. The heart actually starts out as a long tube that twists around itself somehow, it's amazing