In the midst of the sleep deprived haze that goes hand-in-hand with waking for night feeds month after month, tired new mums can be forgiven for deciding their current baby will be their last. It turns out that's just what your bundle of joy has planned, even if they don't realise it.
By robbing mum of a peaceful night's sleep, babies reduce the chance of their parents having the opportunity, or desire, to create a sibling with whom they would have to compete for parental attention, according to Harvard University evolutionary biologist Professor David Haig.
Professor Haig, whose theory is published in the current edition of the journal Evolution, Medicine and Public Health, says while babies are not aware of their intentions at the time, throughout history babies who cried and nursed at night had a survival edge. This could be because their night waking and feeding led to a greater length of time before the arrival of a younger sibling.
"As any parent will affirm, the more children one has the less one can provide for each in purely material terms, but parents are often reluctant to concede that similar trade-offs exist with respect to less material investments of time, care and attention. But an onlooker can testify that a mother encumbered with a babe in arms is less able to grab a toddler as the older child stumbles into danger," Professor Haig wrote.
"Short delays until the birth of a younger sibling are associated with increased mortality of infants and toddlers, especially in environments of resource scarcity and rampant infectious disease."
So in a case of survival of the fittest, babies on the lookout to protect their own wellbeing use whatever means they can to remain the focus of their parents' attention for longer, Professor Haig believes.
The 'waking at night' method of sibling prevention works in two ways. Firstly, it prevents mum and dad from having the time, or libido, required to create another baby. Secondly, the longer and more frequently a baby nurses, the longer it will take for its mother to begin ovulating and become fertile again.
Haig points out that many babies seem to increase their demand for overnight attention and feeds around the six month mark - presumably to curb their parents' baby-making opportunities, just in case mum and dad thought enough time had passed since the birth of their child to start trying for another.
"I am just suggesting that offspring have evolved to use waking up mothers and suckling more intensely to delay the birth of another sibling," Haig told US website NPR. "It is clear that babies can get enough milk even if they sleep through the night. The waking becomes a different issue."
If you ignore the fact that babies could also be waking overnight because they are hot, cold, need a nappy change, or have a sore tummy, then Haig's survival of the fittest theory behind a baby’s after dark attention getting antics makes some sense.
But truth be told it's often the reaction of the baby's father to night time wakings that holds the real power to prevent the creation of any siblings. As any tired mum will tell you, there is no better contraception than a partner who rolls over and snores through their infant's night time cries, only to comment the next morning on how well the baby slept last night.