Newborn cry at night to keep parents away from sex!

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Know why your newborn cries incessantly at 2 a.m., when you may have some plan for love making?

Well, according to a surprising hypothesis, the baby is delaying the conception of a sibling by keeping mom exhausted and not ovulating!

"I am just suggesting that offspring have evolved to use waking up mothers and suckling more intensely to delay the birth of another sibling," said evolutionary biologist David Haig from Harvard University.

In other words, the drive to wake mom (or dad) up is an evolutionary one: Babies that scream and suckle in the early hours are more likely to survive and have children themselves.

There is logic to the idea, though.

"It is clear that babies can get enough milk even if they sleep through the night," Haig told an NPR health blog.

When times are tough - say during food shortages or when infectious diseases are common - babies and toddlers have a better chance of surviving when their parents wait a while before having another child.

Nursing a child, especially at night, seems to hinder many women from resuming ovulation soon after a pregnancy.

"So if a baby can force mom to feed him at night, she may stay infertile longer. And then the baby will have mom and dad all to himself. And a better chance of survival," Haig emphasised.

It is probably not that simple, though, according to anthropologist Holly Dunsworth from University of Rhode Island.

"It is an interesting perspective but it is not the only one," Dunsworth added.

According to Haig, she wants parents to know that children are quite robust and can handle a variety of environments.

"One should not be too anxious with minor variations in parental care," she noted in a study published in the journal Evolution, Medicine and Public Health.