A woman's fertility declines with age, even when she tries to get pregnant through in vitro fertilization (IVF), and a recent study sheds some light on exactly why.
IVF is a procedure in which eggs are taken from a woman's ovaries and fertilized with sperm in a petri dish. Then one or more fertilized embryos are placed in the woman's uterus, where one will hopefully implant and grow into a baby.
For a woman doing IVF with her own eggs, the likelihood she will give birth to a healthy baby declines dramatically with age, from about 50% in her early 30s to below 10% by age 44. But for women who use donor eggs and not their own eggs for IVF, those over 45 years old had kids at about the same rate as women in their early 30s.
The fact that there's such a disparity in live birth rates between women using their own and donor eggs shows that it's the age of the woman the egg comes from that affects those chances more than the age of the woman carrying the baby. That also seems to be why certain pregnancy complications are more common in older mothers.
That's because as a woman ages, her eggs age too. Older eggs are more likely to have an abnormal number of chromosomes, which will prevent them from developing into healthy babies if fertilized. Doctors have long known this in practice, but scientists from RIKEN in Japan and the Karolinska Institutet in Sweden recently figured out what exactly goes on in older eggs that makes them more likely to be abnormal.
Yogo Sakakibara of RIKEN and colleagues watched hundreds of mouse eggs mature through a particular kind of cell division called meiosis, and compared what happened in the eggs of older mice to those of younger mice. Their results, published in Nature Communications, show that a key part of meiosis goes wrong more often in older mice.
At the beginning of meiosis, a cell that will become an egg has four copies of each of its chromosomes. During meiosis, that one cell will divide into four. The point of this complex process is for a mature egg to end up with exactly one copy of each chromosome. This genetic material will provide the mother's half of the child's DNA if that egg is fertilized and grows into a baby.
That one-copy-per-egg is very important, and it requires a very orderly, exact procedure. To make it happen, the chromosomes line up and separate evenly as the cell divides itself a couple times.
That's what the chromosomes should do. But that's not always what happens. When researchers examined the maturing egg cells of older mice, they saw something going very wrong.
In those cells, chromosomes are more likely to separate and step out of line before they should. This means that instead of the mature egg ending up with one copy of a chromosome, it might have two, or none. That'll cause problems: Cells with the wrong number of chromosomes often die or grow much more slowly than normal.
Other things can go wrong with chromosome separation in meiosis, but early separation was the most common reason eggs didn't end up with the right number of chromosomes.
The researchers wanted some preliminary evidence that what they were seeing in mice was also true in humans, so they looked at 16 human eggs donated from women who had done IVF. They found that three eggs, all from women over 35, were predisposed to have this early separation problem. That's a hint that what they found in older mice is a real clue to what's going on in older women.
With this new data, scientists understand just a little more about what happens to older women's eggs that makes it harder for them to have children.
