Fluid on baby's brain in uterus

Sam • Mum of a 2yr old daughter, recovering from a complete molar pregnancy, Adoring fiancé, looking forward to TTC again
So my sister is 20weeks and just had her scan, her GP called and said she needs to go back next week cos their boy has a bit of fluid on the brain, when she asked what is meant for his health the dr wouldn't say anything, told her to wait till the scan, actually he said 'I don't have a crystal ball I can't answer that' asshole!! 
​Anyone ever experienced this or know of someone who has, we don't know what it all means ?