When nausea strikes a pregnant mother it's caused by "greedy little embryos" controlling their iodine, according to a University of Winnipeg professor. 

Biology professor Scott Forbes's research paper "Embryo Quality: The Missing Link Between Pregnancy Sickness and Pregnancy Outcome," recently published in the journal of Evolution and Human Behavior, says nausea and vomiting are signs the embryo is healthy and the mother has sufficient iodine.

"It's simply the best quality embryos are also the ones that can cause pregnancy sickness in their mothers," he said. 

"They are doing it for a rather sinister reason, which is to steal a valuable resource from their mother — stored iodine."

Iodine is an essential micronutrient in the maternal diet and helps the embryo's brain development. 

"Iodine is one of these things a bit like Goldilocks: too little is bad and too much is bad," Forbes said. 

"So pregnancy sickness is really a way of modulating iodine intake."

'Conniving means to get the iodine'

Morning sickness has been a long-standing medical enigma even though two out of three women who are pregnant experience some nausea and vomiting, Forbes said. 

A popular hypothesis was that pregnancy sickness protected the embryo from dietary hazards like bacteria or toxins in food. Forbes says his research proves that's not true.

Instead, it shows it's actually a tug of war for iodine between the mother and embryo, but the embryo uses a hormonal manipulation to hijack its mother's thyroid system and release the iodine, Forbes said.  

"They use conniving means to get the iodine from their mother," he said.

Higher sickness rates benefit embryos

In iodine-rich areas of the world, especially in coastal areas like Japan and South Korea, there are higher rates of pregnancy sickness. This benefits the embryo by reducing the intake of iodine below levels that could be harmful, Forbes said. 

However, close to a third of the global population has an iodine deficiency. Forbes said in iodine-poor environments, there are low levels of pregnancy sickness.

"That's very important because iodine and thyroid hormone is absolutely critical for brain development. In regions of the world where women suffer from severe iodine deficiency, we see cognitive deficits of 12 to 15 IQ points," Forbes said.

Low levels linked to brain damage 

The World Health Organization says iodine deficiency is the world's most prevalent, yet easily preventable, cause of brain damage.

Women who do not get sick are also more likely to lose their pregnancy because of chromosomal defects, he added.

Women should still consult with their doctors about pregnancy sickness, Forbes said. But he hopes his research will lead to routine thyroid testing for pregnant women, a process which is currently controversial.

"An absence of pregnancy sickness tips the balance towards testing for thyroid function. If you have an underactive thyroid, that's easily dealt with," he said.

With files from Leif Larsen