For nearly half a century, the “production line theory” has been used to explain why older mothers experience more miscarriages and infant birth defects. A recent study has debunked this common theory, proving that the science behind the commonly accepted theory just doesn’t add up.

In this study, researchers from Washington State University compared thousands of eggs produced by women of various ages. In the widely known “production line theory,” it was thought that eggs produced by women later in life had weaker connections between chromosomes, which then contributed to increased negative birth outcomes. After the analysis of over 8,000 eggs, no significant difference in chromosome connections was found.

"We do see a pretty high incidence of abnormal cells, but they're just as likely to be happening early as late," explained Ross Rowsey, a doctoral candidate in WSU’s Center for Reproductive Biology, Science Daily reported. "If the production-line hypothesis were true, you'd expect lots of abnormal cells and you would expect them all to be happening late." Rowsey's study showed this was not the case.

The production line hypothesis was first brought to public attention by Alan Henderson and Robert Edwards in 1968. Today, it is the most commonly accepted explanation for fetal chromosome problems in human embryos. This study is not disproving the idea that older moms have higher chances of risky pregnancies and birth abnormalities. This still remains true. In fact, according to co-author of the study, Terry Hassold, “the age of the woman is probably the most important risk factor associated with any human genetic disease.”

Still, if it isn’t abnormal crossovers that are causing the problem in older eggs, then what is? Researchers believe that it’s the faulty chromosomes in older eggs that significantly contribute to problematic pregnancies. Aneuploidy, a condition where there is an incorrect number of chromosomes, is accountable for more than one-third of human miscarriages and congenital birth defects, such as Down syndrome. It is a common event in pregnancy; however, most affected embryos do not survive past the first trimester. This condition is more common in older mothers because “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,” explained Hassold to Counsel Heal.

Source: Rowsey R, Gruhn J, Broman KR, Hunt PA, Hassolf T. Examining Variation in Recombination Levels in the Human Female: A Test of the Production-Line Hypothesis. Cell. 2014.