Researchers have discovered the hormone responsible for , according to a new study. As such, when women are feeling like they need to rest and do not have the energy that they did before becoming pregnant, there is now a very valid reason why sets in much quicker than it previously did.

A team of researchers from the University of Otago have discovered that while pregnant women's bodies are undergoing monumental changes to support carrying a baby to term, their hormones are causing them to feel more tired in the process. Specifically, one particular hormone, prolactin, according to . As such, the hormone that is responsible for has now been linked to as well.

The study, which was published in the journal, stated that prolactin had not been previously suspected as being a culprit in influencing the energy levels of pregnant women. Instead, the uses for prolactin were thought to only affect changes in metabolism, mood, and appetite.

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When women are not pregnant, the body keeps levels of prolactin low. If prolactin levels are found to be high when not pregnant, it can lead to infertility and treatments will be necessary to be able to conceive, according to

During pregnancy, however, prolactin gradually increases until the third trimester. At that time, according to a study published in the , the prolactin levels are 10 times higher than they were at the time of conception. And these increased levels occur to stimulate milk production and get the body prepared to nourish the baby who will be born in a matter of weeks.

To determine that prolactin not only is responsible for creating the milk supply a mother will provide to her baby but also for the energy levels a woman has during pregnancy, researchers observed the cycle of reproduction in mice from pre-conception to the post-weaning phase, according to the . And what they saw astonished them.

Before conception, the mice actively ran on their wheels seven to 13km per day, according to the study. When conception occurred, that distance dropped to 6 km and dropped to 500m by the time of delivery came.

When lactating, the level of activity was between 500m to 1,000m. But as soon as the pups were weaned, per the , the voluntary activity level went back up to seven to 13km per day for the mice. And these were all changes made by the ever-changing levels of prolactin found in the system of the mother mice.

According to Medical Xpress, researchers state that this level of inactivity would have been important in the distant past when resources were scarce and women needed to conserve their energy to bring a healthy baby to term. However, the sedentary lifestyle in more modern times only increases the chances of putting on extra weight during pregnancy, which could lead to obesity after delivery.

It is no coincidence that pregnancy leads to women becoming more tired the closer they get to delivery. But now instead of feeling like the world needs to be conquered when exhausted, knowing that hormones are behind the level of exhaustion felt may be the excuse some women need to slow down for a moment, be present, and enjoy the miracle that pregnancy is.

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