A recent study found that drinking too much water in labor depleted sodium levels in the blood. 

Specialists, from Sweden, conducted a prospective observational study of 287 women at full term (37 completed weeks of gestation). Of the study group, 125 women were first time mothers and 162 had babies before. During labor, the women were allowed to drink water freely, but no solid foods were allowed (in accordance with the hospital’s policy).

Hyponatremia, a fall in plasma sodium levels, during labor was associated with how much water a woman drank and how long labor was. Hyponatremia was significantly associated with the fluid intake, but not with oxytocin administration or with epidural analgesia.

Drinking water in labor, without eating foods, allowed the sodium level to drop. Low sodium blood levels were significantly associated with a prolonged second stage of labor, with instrumental delivery, and with emergency cesarean section performed because of a failure to progress.

The researchers suggest in their paper that, based on their data, hyponatremia “is not uncommon following labor”. A quarter of the women drinking 2.5 liters of water (and remember this is without eating food) developed low sodium blood levels. Women whose labors were 8-10 hours or longer were also more likely to get hyponatremia if they drank only a bit more than 1 1/4 cup (300 ml) of water an hour. 

Some women will drink an electrolyte drink in labor. It can be assumed that this would protect women from low sodium levels, but this wasn't studied. Some electrolyte drinks that are popular among laboring women are Recharge by Knudsen, Emergen-C by Alacer, Gaterade (diluted in half) by Pepsi, Welch's (or other brand) Grape Juice, Hibiscus blossum tea (from bulk herbs), or a recipe, homemade. One serving per 4-8 hours should be sufficient. Hot weather may increase the need for an electrolyte drink. 

An electrolyte drink recipe will include 8 oz. of water, 2-3 tablespoons of lemon juice, a rounded tablespoon of sugar, a pinch of baking soda and a pinch of salt.

The article sited is

Hyponatremia complicating labor

Issue 03: 16 Feb 2009
Source:BJOG: An International Journal of Obstetrics and Gynaecology 2009;116:564-73