When I booked, my place told me 1.5 litres an hour before my appointment. However I only drank 500ml.

When I went for my 12 week scan (at the same place), the sonographer told me a couple of glasses would be enough for me. She said people who drink alot of water have their bladders fill quickly, but they tell everyone 1-1.5 litres because the general population doesn't drink enough water daily.

So I remembered this and only drank enough to make me just need to go to the toilet and at my 18 week scan she said it was perfect