Why pregnant women are obsessed with tidying and nesting: It's all to do with their inner cavewoman Pregnant women also become more selective about the company they keep This may signal onset of labour when it occurs at 40 weeks into pregnancy Obsessive behaviour is the result of a instinct to 'protect and prepare' A pregnant woman's urge to clean, organise and get their life in order is a primal instinct going back thousands of years, according to a new study. Researchers found the obsessive behaviour - known as 'nesting' and characterised by unusual bursts of energy - is not irrational, but 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 just spend time with people they trust, reports the journal Evolution & Human Behaviour. A pregnant woman's urge to clean, organise and get their life in order is a primal instinct going back thousands of years, according to a new study 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. Psychologist Doctor Marla Anderson, of McMaster University in Canada, said: 'Nesting is not a frivolous activity. We have found it peaks in the third trimester as the birth 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 infant. Females of the animal kingdom are all equipped with this same need. Just as you see birds making their nests, mothers to be do exactly the same. Women also become more selective about the company they keep, preferring to just spend time with people they trust, reports the journal Evolution and Human Behaviour They may become a homebody and want to retreat into the comfort of home and familiar company, like a brooding hen. The nesting urge can even be a sign of the onset of labour when it occurs close to 40 weeks of pregnancy. Women have reported throwing away perfectly good sheets and towels because they felt the strong need to have 'brand new, clean' ones in their home. They have also reported doing things like taking apart the knobs on kitchen cupboards, just so they could disinfect the screws attached. Women have discussed taking on cleaning their entire house, armed with a toothbrush. There seems to be no end to the lengths a nesting mother will go. Women have reported doing things like taking apart the knobs on kitchen cupboards, just so they could disinfect the screws attached Cleaning the kitchen cupboards and organising everything by size to the point the silverware patterns match when stacked in the cutlery drawer, and sorting the baby's clothes over and over again, are other common activities. 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 one comparing pregnant and non pregnant women and another tracking participants throughout pregnancy and afterwards. 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 in the third trimester women tell us they are more tired while simultaneously showing an increase in activity,' said Professer Mel Rutherford. So the urge to nest is a very powerful motivating force. The next step is to examine how women may exhibit nesting behaviours when adopting a baby and whether or not expectant fathers demonstrate similar behaviours in the months before a baby is born.