Pregnant women are actually driven to clean and organize because of an adaptive behaviour from humanity’s history, according to a new study from McMaster University.

It's commonly known as nesting, and it’s characterized by unusual bursts of energy and a compulsion to organize the home, which researchers say stems from an evolutionary mechanism to protect and prepare for the unborn baby.

“Nesting is not a frivolous activity,” says Marla Anderson, lead author of the study and a graduate student in the Department of Psychology, Neuroscience and Behaviour.  “We have found that it peaks in the third trimester as the birth of the baby draws near and is an important task that probably serves the same purpose in women as it does in other animals.”

Women also become more selective about the company they keep, researchers say, preferring to spend time only with people they trust.

That all adds up to having control over their environment — and having that control ends up being a key feature of preparing for having kids, including decisions about where the birth will take place and who will be welcome.

“It ties us to our ancestral past,” Anderson said. “Providing a safe environment helps to promote bonding and attachment between both the mother and infants.”

Researchers say that they set out to explore the psychology behind the phenomenon because little scientific research had been done previously on nesting behaviours.

They designed two separate studies: a large online study comparing pregnant and non-pregnant women and a second study tracking women throughout pregnancy and afterwards.

Non-pregnant women acted as the control group. They were quizzed at similar time intervals, using a questionnaire that was developed from interviews with midwives.

Researchers found a paradox: women told them that in their third trimester they are more tired, but still found they were driven to be more active, too.

“So the urge to nest is a very powerful motivating force,” said Mel Rutherford, a professor in the Department of Psychology, Neuroscience and Behaviour.

“It’s part of our human psychology. This is just part of what people do when a birth is imminent,” he said, adding that women who were recently pregnant were really happy to talk about why they “woke up at 2 a.m. with a crazy urge to clean the closet.”

The next step, researchers say, is to examine if women exhibit nesting behaviours if they’re adopting a baby and whether or not fathers might do similar things in the months before a baby is born.

“The big question is — is this entirely mediated by hormones?” Rutherford said. “We don’t know yet.”

The research is published online in the journal Evolution & Human Behaviour.