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Now University of Otago researchers have found a scientific reason why.
Prolactin, the hormone which gears up the body for breastfeeding, also tells the brain to relax rather than run.
Lead author Dr Sharon Ladyman, of the department of anatomy, said the hormone was present in high levels during pregnancy and might account for the common feelings of tiredness and fatigue.
"This hormone is critical for milk production during lactation, and we have previously known that it is also important in promoting interactions between a mother and her baby.
"The fact that it can also influence behaviours such as physical activity during pregnancy had not been previously suspected," she said.
Researchers observed the voluntary use of running wheels by mice through their cycle of reproduction — pre-pregnancy, pregnant, lactation and post-weaning.
Before mating, mice were voluntarily running on the wheel for a distance of between 7km and 13km each day.
On day 1 of pregnancy, that distance dropped to about 6km a day and continued to decrease steadily during pregnancy, eventually dropping to less than 500m at the end of pregnancy.
The distance stayed low during lactation, but post-weaning it spiked back to between 7km and 13km a day.
"Despite pregnant women being extremely open to changing their behaviour for the benefit of their babies, many do not achieve the recommended levels of physical activity during pregnancy," Dr Ladyman said.
"We have now provided a potential biological explanation for this: their bodies are telling them not to engage in physical activity."
It had previously been known that high levels of prolactin during pregnancy had multiple actions to help mothers prepare for the changes happening in their bodies, such as changes in metabolism, appetite, the way they dealt with stress and changes in mood.
While most of these changes were beneficial, they were "likely a relic of our evolutionary past, when resources were scarce and needed to be preserved for use in growth of the baby", Dr Ladyman said.
"But in our modern obesogenic environment, such changes have become maladaptive, contributing to excessive weight gain during pregnancy with the associated increased risk in pregnancy complications."
By improving the understanding of the biological changes that were happening, she hoped women would be able to be better advised on what to expect, Dr Ladyman said.