from Pregnancy Weekly
Around the fifth month of pregnancy, the "nesting" instinct can set in. This is an uncontrollable urge to clean one's house brought on by a desire to prepare a nest for the new baby, to tie up loose ends of old projects and to organize your world.
Females of the animal kingdom are all equipped with this same need. It is a primal instinct. Just as you see birds making their nests, mothers-to-be do exactly the same thing. The act of nesting puts you in control and gives a sense of accomplishment toward birth. You may become a homebody and want to retreat into the comfort of home and familiar company, like a brooding hen. The nesting urge can also be seen as a sign of the onset of labor when it occurs close to 40 weeks of pregnancy.
Nesting brings about some unique and seemingly irrational behaviors in pregnant women and all of them experience it differently. Women have reported throwing away perfectly good sheets and towels because they felt the strong need to have "brand new, clean" sheets and towels 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 to the knobs. 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 to prepare for her upcoming arrival.
This unusual burst of energy is responsible for women ironing anything in the house that couldn't out run them. Being preoccupied with ant killing, squishing them one at a time for weeks on end. Packing and unpacking the labor bag 50 times. Cleaning the kitchen cupboards and organizing everything by size to the point that you make sure the silverware patterns match when it's stacked in the cutlery drawer. Sorting the baby's clothes over and over again is a favorite theme. Taking them out of the drawers and re-folding them, putting them away and doing it over and over again. Nesting will provide interesting stories for years to come.