Taking Vitamin B3 could prevent miscarriages and birth defects, a study on mice suggests.
Researchers from the Victor Chang Institute in Sydney called it "a double breakthrough", as they found both a cause and a preventative solution.
With 7.9 million babies born each year with a birth defect worldwide, the team hopes the benefits are wide-reaching.
But an expert said the findings "cannot be translated into recommendations" for pregnancy.
The researchers analysed the DNA of four families where the mothers had suffered multiple miscarriages or their babies were born with multiple birth defects, such as heart, kidney, vertebrae and cleft palate problems.
They found mutations in two genes that caused the child to be deficient in a vital molecule known as Nicotinamide adenine dinucleotide (NAD), which allows cells to generate energy and organs to develop normally.
Lead researcher Prof Sally Dunwoodie replicated these mutations in mice but found they could be corrected if the pregnant mother took niacin (vitamin B3).
"You can boost your levels of NAD and completely prevent the miscarriages and birth defects. It bypasses the genetic problem," she said. "It's rare that you find a cause and a prevention in the same study. And the prevention is so simple, it's a vitamin," she said.
Dr Katie Morris, an expert in maternal foetal medicine at the University of Birmingham, said: "While exciting, this discovery cannot be translated into recommendations for pregnant women, who at most may be deficient in vitamin B3.
"The doses used in this research were 10 times the recommended daily doses for supplementation in women."
She said the side-effects of this high dosage are not known, with pregnancy complications often occurring because of the complex interaction of a number of factors.
Prof Jean Golding, from the University of Bristol, called it a "solid piece of work" but cautioned against extrapolating too much from the findings, because they were based on the genetics of four families and mice.
For now, Prof Dunwoodie recommended pregnant women take a pregnancy-specific multivitamin, which includes the advised 18 milligrams of niacin.
"But, we're not all the same in how we absorb nutrients," she said, adding that body mass index and diabetes can influence how a woman produces NAD.
She added: "We don't know who these women are that don't make sufficient levels, so that will be the next thing to study."