The recommendation when pregnant is 1.5 times what you would need when not pregnant. So at least in the US, the water recommendation for a non-pregnant, normal weight adult is 64 fl oz (1.9L), so when pregnant you should try for at least 96 fl oz (2.8L) per day.

My OB was very clear that I should drink a gallon per day throughout pregnancy. That’s 128 ounces. It’s the one of the only things she was really serious about so I’ve been diligent. Not sure if it’s coincidental, but I’ve also had very minimal symptoms 🤷🏻♀️