Because of egg quality and chromosomes, a best-case generalization is that of 12 eggs retrieved in IVF only four will likely be healthy embryos
Patients are often puzzled by the fact that many of their eggs retrieved during in vitro fertilization (IVF) do not become embryos. What many people don’t realize is that embryo development in humans is incredibly inefficient. This is ultimately due to egg quality factors, which refers to how many eggs remaining in a woman’s ovaries are still chromosomally normal. Fertility specialists often call these “viable” eggs.
These single-cell eggs have to stay viable for years – decades even – before they might get their one chance to be fertilized and become an embryo. That’s because women are born with all the eggs they will ever have in their lifetime and they never create new eggs.
Advanced maternal age means more abnormal chromosomes
As time passes, chromosomes in the egg can start to break down, or even stick together. Sperm can still fertilize these eggs, but the embryo will have too many or too few chromosomes. Embryos with abnormal chromosome numbers, known as aneuploidy, typically either result in no pregnancy as many will not implant, or miscarriage if the embryo successfully implants but does not grow much beyond 8 weeks of pregnancy.
Here’s a guide to age and chromosome abnormality in eggs:
- For women under 35, about 70% of eggs are chromosomally normal.
- For women at age 37, about 40% of eggs are chromosomally normal.
- For those age 40, about 25% of eggs have normal chromosomes.
- At 45 about 5% of eggs are chromosomally normal.
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Statistics of retrieved eggs in IVF & healthy formation of embryos
Here is an example I give my patients of typical statistics for an IVF cycle. The average woman gets about 12 eggs at an egg retrieval. Of these, about 10 eggs are expected to be mature. Only mature eggs can go on to embryo formation after being fertilized with sperm, and only 80% of mature eggs fertilize. So now about eight of the 10 mature eggs become embryos.
Next, the embryos have to survive and develop over the next five days (whether they are in the embryology lab incubator or in a woman’s uterus through a naturally conceived pregnancy). Only half of embryos at best survive, so now we are down to four embryos.
Female egg quality and the chance for live birth
Now the statistics on egg quality come into play affecting embryo quality.
- For a woman under 35, we’d expect 70% of the embryos to be chromosomally normal, or about three using our example.
- For a woman at age 40, only one of the four will be chromosomally normal.
Chromosomally normal embryos have a 65% chance of resulting in the live birth of a baby. So, the woman under 35 has three tries, each with a 65% chance of success. A woman at age 40 has one try. Often women in their late 30s and 40s must undergo a second or even third egg retrieval in an effort to make more chromosomally normal embryos, if the first embryo is not successful.
It is important to note that our ability to treat chromosome problems in eggs is still a long way off from being standard clinical practice. Fertility medicine has yet to develop the means to overcome the effect of aging on egg quality.
We can’t encourage the good eggs to become healthy embryos
IVF is an emotional rollercoaster. Women going through infertility have been reported to have higher levels of anxiety and depression when compared to patients with cancer going through cancer treatments. Ultimately the stress is due to lack of control.
We have no control over which eggs in the ovaries grow in an IVF cycle. Each woman has viable and nonviable eggs, and there is no way for us to encourage the “good eggs” to grow and prevent the “bad eggs” from being harvested. The statistical chance of success in each IVF cycle depends on the individual’s age, ovarian reserve and specific diagnosis for infertility.
Some women become pregnant on the first try. Others will have to try more than one or two cycles before they succeed. This lack of control and lack of knowing what the future will bring leads to the incredible stress we see women go through in fertility treatment.