The Correlation Between Drinking Fluids In Pregnant And Lactating Women

The human body is composed of mostly water, which supports various metabolic processes, but we often forget how essential water truly is. Recent evidence suggests that drinking enough water to keep water turnover high (i.e. drinking an adequate amount of fluids so urine is less concentrated) might help prevent issues like repeated kidney stones or urinary tract infections. There’s even evidence to suggest a small amount of dehydration can impact cognitive performance and mood.

These relationships, however, have been studied in your average man or woman— but are they the same during pregnancy or nursing? Water takes on new roles during this time, as it’s used to expand the mother’s blood volume, develop the placenta that delivers nutrients to the baby, and fill the amniotic sac with fluid to provide the baby with a gentle environment. While breastfeeding, water helps mom provide about 700 ml – or about 3 glasses (8oz each) – of breastmilk each day to her baby. And physiologically, lots of changes happen at the hormonal level to support these functions, which also impact the way the body balances fluids.

Research has shown that for women who are not pregnant or nursing, characteristics about their urine, like the volume, concentration, and color, inform us about how much fluid the woman is drinking and if that volume is sufficient. Characteristics of the urine describe the net balance of the hydration process after considering all of the fluids going in and going out of the body. The more fluid one drinks, the higher the urine volume, the lower the concentration, the lighter the color, and ultimately, the better the chances are that the person is consuming enough fluid to stay well-hydrated. However, none of the studies evaluating these relationships included pregnant or nursing women. Given the physiological changes that occur during that time, we didn’t know if how much fluid pregnant or nursing women drink are reflected by urine characteristics in the same way.

We asked moms-to-be and a control group of women who weren’t pregnant to collect their urine and record the fluids they consumed for a day, five separate times. Pregnant women did this at the end of each trimester, and at 3 and 9 weeks postpartum while nursing; the women who weren’t expecting also collected these samples over a similar time period. In the lab the next morning, we analyzed the women’s fluid records to determine total fluid intake (the volume of beverages consumed), took a blood sample to measure concentration and other markers of hydration, and measured a few different qualities of the 1-day urine samples they collected—volume, concentration, and color. With this information, we examined if the characteristics that reflected total fluid intake in non-pregnant women were the same or different from women who were pregnant or nursing, and we reported in the European Journal of Nutrition.

For all of the women we studied, total fluid intake for the day wasn’t related to any marker of hydration in the blood the next morning, but total fluid intake was related to 24h urine volume, markers of urine concentration, and urine color. Women who consumed higher volumes of fluids tended to produce higher volumes of urine with lower concentrations and lighter color. Additionally, these relationships were similar between all three groups of women (pregnant, nursing, or not expecting).

We also wanted to learn about the relationship between breast milk volume and total fluid intake. To do so, we asked the nursing mothers to weigh their baby before and after each feeding to estimate the volume of milk provided to the baby over the course of the same day in which they collected their urine and recorded their fluid intake. The volume of milk provided in one day, however, was not related to total fluid intake — this volume was approximately the same across a wide range of fluid intake values, from as little as 50 ounces to as much as about 120 ounces of fluid consumed per day. That consistency despite variable intake is great news for the baby’s nutrition and hydration, but this prompted us to take a closer look at the urine characteristics of the mother to get a sense of their net hydration process.

Nursing moms who had higher total fluid intakes tended to have lower urine concentrations than non-lactating women consuming the same amount of fluid. But in lactating women consuming a low total fluid intake, urine concentrations tended to be slightly higher than their non-lactating counterparts. This appears to be another example of how the mother’s body protects breastmilk so that it’s exactly right for the baby. This doesn’t seem to be an issue when consuming a total daily fluid intake in the normal or slightly high range, but when intake is low, protecting the volume of breastmilk provided to the baby might come at a cost to the mother. The loss of water via breastmilk appears to be an additional challenge to the mother’s fluid balance, especially when fluid intake is low.

To summarize, this study provides evidence that the markers we typically use to evaluate hydration are also valid in pregnant and lactating women.  Since water takes on new roles during pregnancy and lactation, it’s important to consider these additional challenges, such as maintaining appropriate breastmilk volume and concentration and provide guidance to mothers who may be more susceptible to improper fluid balance.

Future research can further our understanding of how practical everyday tools, like the assessment of urine color, can be used to assess daily fluid intake. In addition, research on behavior change interventions can help figure out how to promote better fluid intake habits when necessary. By developing these strategies, we can help everyone—including pregnant and nursing women—practice healthy hydration

These findings are described in the article entitled Relationships between hydration biomarkers and total fluid intake in pregnant and lactating women, published in the journal European Journal of Nutrition. This work was led by Amy McKenzie from the University of Connecticut.