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The "Production-Line Hypothesis" is a 46-year old belief that the eggs that a woman produces early on in life have better connections between chromosomes. As a woman ages, weaker connections, or crossovers, can lead to miscarriages and developmental abnormalities. The hypothesis also states that the older a woman is, the more likely she will have faulty chromosomes In a new study, researchers tested the veracity of this hypothesis and found no evidence supporting the theory regarding the relationship between a woman's age and the connections in her chromosomes.

For this study, the team of reproductive biologists from Washington State University compared thousands of eggs that were produced during different ages. The researchers counted the actual chromosome crossovers. They discovered no differences in the numbers of crossovers between eggs that were produced early on in life and eggs that were produced at a later stage.

"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 Ross Rowsey, a doctoral candidate in WSU's Center for Reproductive Biology reported in the press release. "We do see a pretty high incidence of abnormal cells, but they're just as likely to be happening early as late."

The researchers explained that instead of chromosome crossovers, faulty chromosomes did seem to have an effect on the risk of miscarriages and developmental abnormalities. The researchers found that aneuploidy, which occurs when there is an incorrect number of chromosomes, could be tied to more than one-third of miscarriages and birth defects. The researchers stated that older women tend to have aneuploidy more often than young women.

"The age of the woman is probably the most important risk factor associated with any human genetic disease," said co-author, Terry Hassold, a WSU professor of reproductive biology. "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."

The findings were published in the American Journal of Human Genetics.