Maternal dehydration

I seem to have mentioned in a couple of entries how easily I’m feeling thirsty when exercising. As we know, this is a symptom of dehydration – I meant to address the topic of maternal dehydration some time ago, but at the time got distracted by the whole Pelvic Girdle Pain issue instead…

My previous entry looking at thermoregulation during pregnancy showed that the pregnant body is pretty amazing at keeping a constant temperature for the baby, despite mum’s own hyper- or hypothermia. Is it the same for maternal dehydration; can mum still keep bub well-hydrated (whatever that means when you’re surrounded by fluid in the womb?) or does baby suffer when mum gets dehydrated?

Pregnant women can lose fluids via a variety of mechanisms, with heat, sweat and gastrointestinal (eg. Vomiting) being the main ones. This fluid loss results in an increased osmolality of the plasma (basically how concentrated your blood is). One study looked at the effect of dehydrating, and then rehydrating, a group of pregnant ewes (poor furry friends). It found that in response to maternal dehydration, the foetuses had more concentrated serum, contraction of their blood volumes, increased AVP secretion (a hormone that the body secretes when dehydrated in an attempt to conserve water) and the production of more concentrated urine. Interestingly, some of these changes persisted even after the mothers were rehydrated again, showing that the foetuses had prolonged responses to maternal dehydration. Bottom line: if mama dehydrates like a tasty Chinese snack (yes I’m thinking wah mui’s) – even when she’s rehydrated, it’s going to affect baby.

Ok so there is some evidence pointing to the deleterious (long word for the day) effects of maternal dehydration on the baby. As we should all know by now, baby is suspended in amniotic fluid inside the womb, which is produced by both mother and baby (eg. baby lung fluid and urine production – yes, poor baby is swimming in his or her own pee). Ew, back to the studies… This had led scientists to look at whether maternal dehydration has an effect on amniotic fluid production and composition. In another study, the poor pregnant ewes (again!) were dehydrated to see the effect on amniotic fluid. It was found that in the event of maternal dehydration, the concentration of the amniotic fluid increased, and the volume of the amniotic fluid decreased. Bottom line: mum’s dehydration makes the environment baby is living in smaller volume and stronger concentration, which is not great.

So please make sure you remain well-hydrated during your pregnancy, especially when exercising. There are no universally accepted guidelines for the correct amount of fluid intake so I guess we can use common sense here: drinking enough water so that your urine is not strong smelling and it should be pale yellow in colour – if I know I’m going to be exercising for quite a while, I make sure it is as near clear as possible in colour before starting exercise, as it’s often difficult to replace your fluids sufficiently during a long exercise session. Don’t wait until you’re thirsty before you think about replacing your fluids during exercise, and keep rehydrating once exercise is over.


1. Ross MG, Sherman DJ, Ervin MG, Castro R, Humme J. Maternal dehydration-rehydration: fetal plasma and urinary responses. Am J Physiol. 1988 Nov;255(5 Pt 1):E674-9.
2. Schreyer P, Sherman DJ, Ervin MG, Day L, Ross MG. Maternal dehydration: impact on ovine amniotic fluid volume and composition. J Dev Physiol. 1990 May;13(5):283-7.

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