TUESDAY, July 8, 2014 (HealthDay News) -- New research rules out a leading theory on why older women are at greater risk for having a miscarriage or a child with birth defects.

For almost five decades, it's been widely believed that the first eggs produced in a female's fetal stage have better connections between chromosomes. These connections are known as chromosome crossovers. This idea is known as the "Production-Line Hypothesis."

However, scientists at Washington State University (WSU) found that eggs produced early in the fetal stage are no different from those produced later.

"If the production-line hypothesis was true, you'd expect lots of abnormal cells and you would expect them all to be happening late," said the study's author, Ross Rowsey, a doctoral candidate in WSU's Center for Reproductive Biology in a university news release. "We do see a pretty high incidence of abnormal cells, but they're just as likely to be happening early as late."

Humans are born with 23 pairs of chromosomes -- one set from mom and one from dad. Each chromosome carries specific genetic material that defines who you are, such as whether or not you're male or female.

The production-line hypothesis was developed in 1968. Since then it has become one of the most commonly used explanations for why babies are born with the incorrect number of chromosomes -- an abnormality known as aneuploidy, the study authors said.

"The age of the woman is probably the most important risk factor associated with any human genetic disease," said the study's co-author, Terry Hassold, a WSU professor of reproductive biology in a news release. "It's an extraordinary complication to human reproduction. By the time a woman is in her 40s, it's likely the majority of her eggs don't have the right number of chromosomes. And if you don't have the right number of chromosomes, you'll either have a miscarriage or a congenital disability."

Another longstanding theory is that as women get older, their eggs have more abnormal chromosomes, which result in miscarriages and birth defects.

To further investigate these ideas, the study's authors analyzed more than 8,000 eggs taken from the ovaries of almost 200 second-trimester fetuses from elective abortions.

The eggs were treated so that the proteins at chromosome connections or crossovers would become florescent. This allowed the researchers to count and examine them.

Although there was variation among the women, the study, published in the July 3 issue of the American Journal of Human Genetics, revealed there was no link to their age.

"There have to be other factors involved," Rowsey said. "The abnormal crossovers can't be explaining all of it."

The researchers concluded there are three stages at which problems could occur: when the chromosome connections are formed, during the long wait before ovulation when proteins must remain intact for years, and during ovulation and fertilization.

"I don't know a ton about protein stability," Rowsey said, "but it seems to me like a single protein sitting there for 40 years is highly unlikely. But studies from model organisms show that those proteins aren't replenished over time. I'm really interested to know what's going on in that time."

More information

The U.S. Centers for Disease Control and Prevention provides more information on birth defects linked to maternal risk factors.