If you opted to test your embryos for chromosome abnormalities, you hope to
get a result which is normal. Transferring abnormal embryos does not result in live births.
What if you get a result that is both normal and abnormal? Stay tuned and I will explain
First a little background. A normal egg is one cell which contains 23 chromosomes. A normal sperm
is one cell that contains 23 chromosomes. After a sperm penetrates the egg, we end up with an embryo
which has 46 chromosomes. If all goes right, that fertilized egg or zygote should start to divide.
First two cells. Then 4 cells, 8 cells and so on. Each time the cells divide, the chromosomes
are duplicated. In an 8 cell embryo, each of those cells should still have 46 chromosomes.
Sometimes however, mistakes are made during cell division. When these mistakes happen, if a cell
divides, the chromosomes may duplicate but not end up evenly divided into the two daughter cells.
Here is one easy example. Due to a mistake, instead of both of these cells ending with
46 chromosomes, one ended up with 46 chromosomes and one ended up with 47 chromosomes. When those
abnormal embryos continue to divide, you may end up with a bunch of cells that have 47 chromosomes.
The current method used to test embryos is to wait until the embryo has reached
the blastocyst stage when the embryo has over
100 cells. About 5-10 cells are removed. This is called a trophoblast biopsy. Those cells
are then tested to determine whether the cells have the correct number of chromosomes.
If all of the cells in the embryo are normal, the biopsy result will come back normal. If all of the
cells in the embryo are abnormal, then the biopsy result will come back abnormal. But what if some
of the cells are normal and some are abnormal? In that case, the biopsy result may come back mixed,
with some percentage of the cells coming back abnormal. Scientists call these mosaic embryos.
What to do with these “mosaic” embryos has been a source of controversy in IVF for several years.
Initially, studies seemed to indicate that transferring mosaic embryos
would result in lower pregnancy rates and higher miscarriage rates. There was also real concern
about babies born with birth defects. These studies had a number of problems, however, which
made making definite conclusions very difficult. For example, mosaic embryos in these studies
were used only as a last resort when no other good embryos were available.
Poorer outcomes may therefore have been due to lower quality embryos
rather than mosaicism. Some of the older testing methods were also not very accurate.
Recently, however, scientists published the results of what is called a blinded non-selection
study. Here is how it worked. Couples undergoing IVF had their embryos biopsied and tested.
The testing lab reported the embryos as either normal or abnormal. Secretly, however,
they divided the normal embryos into those that were truly normal and those with mosaicism. Low
mosaics had 20-30% abnormal cells. Medium mosaics had 30-50% abnormal cells. Neither the doctors,
the embryologists nor the patients knew if any of the “normal” embryos were mosaics.
This is the “blinded” part. Those patients then had transfer of a single embryo. This is the
non selection part. Mosaic embryos would be just as likely to be selected for transfer as normal
embryos. The actual results were not revealed until later, once the treatments were over.
What were the results? Over 1600 embryo transfers were performed.
They compared the outcomes of three groups: Normal embryos,
Low mosaic embryos and medium mosaic embryos. Four outcomes were compared. First, they found
that the chance for a positive pregnancy test after the embryo transfer was no different.
Each group had about a 55% chance of a positive pregnancy blood test. Second, they analyzed the
rate of biochemical pregnancy loss. This is when a patient had a positive pregnancy test but lost
the pregnancy before it was seen on ultrasound. As you can see, the three groups were similar. Third,
they looked at the later miscarriage rate. These are miscarriages that occured after a
pregnancy was seen on ultrasound. Again, all three groups were similar. Finally,
they looked at the live birth rate. The live birth rate per transfer in each group was about 42%.
This demonstrates very powerfully that the transfer of a low or medium mosaic embryo
has the same chance for live birth as a normal embryo.
There was one other outcome that this study looked at.
They examined the babies born and did a sensitive genetic analysis
on 10% of them. They were again unable to find any difference between the three groups.
Our InfertilityTV bottom line is this: We now have very robust data that low
and medium mosaics have the same reproductive potential as normal embryos.
In 2022, it is acceptable to choose these embryos for transfer