Ideally you will have been drinking plenty of water throughout your pregnancy, so when you go into labour you will be fully hydrated. Eight x 8 ounce glasses of water each day is a good goal, with more if you are exercising.
If you go into labour fully hydrated your body will work more efficiently. Sweating (and sickness) during labour will dehydrate the body - there is no doubt labour is thirsty work! Being fully hydrated before labour starts will improve muscular function and cellular activity and prevent what is known as ‘chronic dehydration’.
It can take several weeks to reach optimal hydration - and 'pre hydration' is easier and safer than 're-hydration'. Just a 2% dehydration in the body can affect physical performance by as much as 10%.
Not drinking enough water and being dehydrated in labour causes your uterus to contract less efficiently, it will detrimentally affect your energy levels, concentration and focus. A December 2010 study in the American Journal of Obstetrics found that hydration during labour was correlated to more effective and shorter labours, along with less risk of induction and/or caesarean
Read more on water here