Water UK

Water requirements in pregnant women

Pregnant women have a slightly increased water requirement because of expanding extracellular fluid space, the needs of the fetus and the amniotic fluid. This has been calculated to be an 30ml extra fluid per day - although the actual amount is likely to vary due to increased heat production and perspiration, particularly during the summer. Lactating women must also replace the fluid lost in breast milk - calculated to be between 750ml and 1000ml per day.(i) The World Health Organization recommends that pregnant women should have 4.8 litres per day and that 5.5 litres are required per day to meet the needs of lactation.(ii)

Amniotic fluid

Oligohydramnios is a condition where there is too little amniotic fluid surrounding the baby in the womb. As a result, the baby may receive too few nutrients and oxygen, and may not be able to turn to the cephalic position (head first) before birth. Such women may suffer from a number of problems in pregnancy, including malpresentation (eg breech), umbilical cord compression, concentration of meconium in the liquor and difficult or failed external cephalic version (manual manipulation to turn a breech fetus to the head down position. Ultrasound visualisation of the baby may also be difficult. Extreme oligohydramnios in early pregnancy can lead to impaired fetal lung development.

A Cochrane review conducted by Hofmeyr and Gulmezoglu found that simple maternal hydration was an effective way of increasing amniotic fluid volume. The review found that to increase the volume of fluid surrounding their baby, women should drink extra water - usually two litres over two hours - although it was noted that drinking two litres of water in a relatively short period of time may be unpleasant and difficult for women and that if the mother experienced difficulties, this could be spread over a longer time. The review concluded that maternal hydration may therefore be beneficial in the management of these problems.(iii) In another study, Brace found that changes in amniotic fluid volume may be the consequence of the existing state of hydration of the fetus and, therefore, abnormal fetal fluid balance may be influenced by the mother.(iv)

Thirst in pregnancy

The thirst threshold is set at lower levels in pregnancy and in the luteal phase of the menstrual cycle (ie women feel thirsty when they are less dehydrated). This may contribute to fluid retention in these situations.(v)

Effects of caffeine

The risk of low birth weight and spontaneous abortion increases with increasing maternal caffeine intake during pregnancy. The Food Standards Agency considers it prudent to assume that caffeine intakes above 300 mg/day shows a plausible association with low birth weight and spontaneous abortion, given the available evidence from studies in experimental animals and epidemiological studies. Intake of 300 mg/day caffeine is equivalent to four cups of instant coffee or about six cups of tea, assuming average caffeine contents. For caffeine intakes of 150 to 300 mg/day there is less evidence for an association.(vi)

Last updated: January 2004