One chart reveals the key reason it's harder for women to have babies when they're older
declines with age, even when she tries to get pregnant through in vitro fertilization (IVF).
Age is, in fact, the most important factor influencing the success of IVF when a woman uses her own eggs, according to the Society for Assisted Reproductive Technology, the professional organization for IVF clinics.
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.
But for a certain subset of women doing IVF, their likelihood of having a baby from the procedure does not change with their age.
These women shed light on why it is older women are less likely to get pregnant in general, and it's not because their bodies can't handle it. It's because of their eggs.
For women who used 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, as shown by the blue line in the graph below. (The wild-looking fluctuations for women below age 30 is likely statistical noise because very few women younger than 30 do IVF with donated eggs.)
That's a sharp contrast to women who used their own eggs (orange line), as you can see in the graph:
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.
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'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. When older women use eggs from a younger donor, they're able to get around that effect of aging.
It seems that the ability of the uterus to carry a fetus is not much affected by age, even past menopause. In a study of women over 50 years old doing IVF with donated eggs, researchers found they were just as likely to get pregnant as younger women. All is not quite equal, however. Certain complications to pregnancy were more common for the older women, and they were also more likely to give birth by Cesarean section.
Still, the data shows that it's the age of a woman's eggs that limits her fertility, not the age of the rest of her body.