The overwhelming urge that drives many pregnant women to clean, organize and get life in order—otherwise known as nesting—is not irrational, but an adaptive behaviour stemming from humans' evolutionary past.
Researchers from McMaster University suggest that these behaviours—characterized by unusual bursts of energy and a compulsion to organize the household—are a result of a mechanism to protect and prepare for the unborn baby.
Women also become more selective about the company they keep, preferring to spend time only with people they trust, say researchers.
In short, having control over the environment is a key feature of preparing for childbirth, including decisions about where the birth will take place and who will be welcome.
"Nesting is not a frivolous activity," says Marla Anderson, lead author of the study and a graduate student in the Department of Psychology, Neuroscience & 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."
"It ties us to our ancestral past. Providing a safe environment helps to promote bonding and attachment between both the mother and infants," she says.
Since little scientific research had been done previously on nesting behaviours, the scientists set out to explore the psychology behind the phenomenon.
They designed two separate studies: a large online study comparing pregnant and non-pregnant women and a longitudinal study tracking women throughout pregnancy and into the postpartum period.
Non-pregnant women—who acted as the control group—were compared at similar time intervals, using a questionnaire which was developed, in part, from interviews conducted with midwives.
"One of the apparent paradoxes of nesting is that in the third trimester of pregnancy women tell us that they are more tired while simultaneously showing an increase in activity," explains Mel Rutherford, a professor in the Department of Psychology, Neuroscience & Behaviour at McMaster University. "So the urge to nest is a very powerful motivating force."
The next step, say researchers, is to examine how women might exhibit nesting behaviours when adopting a baby and whether or not expectant fathers might demonstrate similar behaviours in the months before a baby is born.
The research is published online in the journal Evolution & Human Behavior.