Research conducted in part by the University of Waterloo shows babies whose mothers consumed marijuana while pregnant had an improved type of vision by age four.

The results are part of a longitudinal study, looking at the health of 145 children from the United States and New Zealand. All but 25 of the children had been exposed to methamphetamines and other drugs — like alcohol, cigarettes and marijuana — in the womb.

"They were assessed at birth, at six months and at two years," explained researcher Ben Thompson, an associate professor in the University of Waterloo's department of optometry and vision science.

When then children reached four and a half months, Thompson and researchers with Auckland and Brown University assessed their global motion perception, which is a person's ability to track moving objects.

Their findings were surprising, Thompson said:

Both alcohol and marijuana exposure affected vision at age four.

When children were exposed to both marijuana and alcohol, they seem to cancel out each others' effects.

Marijuana exposure made children respond to this test 50 per cent better than in comparable children.

Overall, drugs had 'negative impact'

Thompson cautioned these results come from just one exam and the fact remains that drugs during pregnancy are bad for a child's development.

"These two drugs changed the way these babies' brains were developing," says Thompson. "They had four-and-a-half years for their brains to normalize but they still had these noticeable differences."

"Outside vision, it's well understood that cannabis exposure has a negative impact on development."

He says the study gives researchers a better understanding of how alcohol and marijuana interact with the developing brain, putting them in a better place to understand what kind of help affected children will need as they mature.