Because our eggs have been with us since before we were born, they’re exposed to all sorts of damaging, but mostly unavoidable, influences throughout our lives: fevers, infections, stress, toxins, free radicals. And because human cells, including eggs, are fragile, this exposure can lead to little “mistakes” in our egg cells’ DNA, called chromosomal abnormalities. Egg quality is fairly black-and-white—either an egg is genetically “normal” (euploid) or it’s not (aneuploid), and as women age, a higher and higher percentage of their eggs are abnormal.
Since DNA is like an instruction manual for our cells, any damage to it can prevent that cell from doing what it’s supposed to do—which, in the case of the egg, is make a healthy baby. An abnormal egg cell typically doesn’t fertilize at all, but in the rare case it does, it can result in miscarriage or genetic disorders like Down syndrome—which is why we see those conditions much more often in pregnant women over 35.
There’s no test for egg quality. The only way to know if an egg is chromosomally normal is to attempt to fertilize it, and, if fertilization is successful, to perform a genetic test on the embryo. But because damage to our DNA happens naturally over time, your age can give doctors a fairly accurate picture of what percentage of your eggs are normal.
Studies of women undergoing fertility treatment have determined that, while women doing in vitro fertilization with their own eggs of the same age experience a significant decrease in success rates, for participants using donor eggs from a younger woman the pregnancy rate was steady across all age groups: 51%. This confirmed an important fact: because age directly correlates with egg quality, it’s the age of the egg that matters most. Yes, there are some risks associated with carrying a pregnancy at what’s called “advanced maternal age” (namely a slightly higher risk of gestational diabetes and preeclampsia), but for the most part, it’s a young, healthy egg that makes for a healthy pregnancy.