I Have a Mosaic Embryo—What Do I Need to Know?

You probably expect the word “mosaic” to show up when you’re searching Pinterest for a new art project to brighten your bedroom walls…not so much when you’re sitting down with your fertility specialist to talk about your embryos. But if you just got the news from your fertility clinic that you’ve got a mosaic embryo, you know they’re not talking about aesthetics. 

So, what is a mosaic embryo, and what does this mean for your chances of having a healthy baby?

What is a mosaic embryo?

Let’s cut through some of the big words that might have been floating around in that doctor’s office, shall we?

A mosaic embryo is an embryo that has what Emily Jungheim, MD — a reproductive endocrinologist at Northwestern Medicine — calls two more “chromosomally distinct cell lines.” Yes, we know, we said we’d cut out the big words. We’re getting there, and this will all make sense in a moment.

The way Jay Flanagan, MD, health expert for the National Society of Genetic Counselors and a clinical associate professor at the University of South Dakota, explains it, embryos are made up of two parts: 

1. Inner cell mass (ICM). That's where the baby comes from.
2. Trophectoderm (TE). That’s where the placenta comes from.

Genetic testing of embryos typically occurs during the in vitro fertilization process about five to seven days after an egg is fertilized — when the egg has grown to what’s known as blastocyst stage. That’s when your doctor will sample some cells from the TE for testing. If some (but not all) of the cells they test have different numbers of chromosomes from the magic “normal” number — 46 — the lab report will describe the embryos as mosaic.

“Think of a mosaic table, they have lots of different rocks or shells,” Flanagan explains. “A mosaic embryo has more than one cell line.”

So how is a mosaic embryo different from any other embryo?

If you look at one of the mosaic tables Flanagan described, you’ll see all sorts of different shapes and sizes of rocks or shells. The same goes for embryos and the cells they’re made from. So you could have an embryo that’s made up of:

Only aneuploid cells

Human cells typically have 46 chromosomes, Jungheim explains, but aneuploid cells have what the doctors call an abnormal number. If your tested cells are all aneuploid, you have what doctors call an “abnormal” embryo, and you’ve also got a much higher chance for failed implantation, miscarriage, or — in some cases — having a baby with developmental or intellectual concerns, Flanagan says.

Overall, most fully aneuploid embryos will either not implant or if they do implant, they will eventually stop developing and result in a miscarriage. A small number of chromosomal imbalances (i.e. having 3 copies of chromosome 21) have the potential to turn into a liveborn baby, but these children will have significant medical issues, like Down Syndrome.

Only euploid cells

Euploid cells are the opposite of aneuploid. They’ve got just the right number of chromosomes — 46. If your test shows only euploid cells, your embryo is not considered mosaic. It’s important to know all euploid cells don’t automatically mean the embryo will result in pregnancy, Flanagan says, but a higher percentage will.

A mix of aneuploid cells and euploid cells

If you’ve got a mix of both kinds of cells, you’ve got a mosaic embryo. And that’s not necessarily a bad thing. It’s possible that, in all likelihood, some of us are even mosaics with more than one cell line, but most of our cells are normal. In other words? Mosaic embryos are…well…not necessarily abnormal.

Should I transfer a mosaic embryo?

This is the question of the hour, and we wish we could just make up your mind for you, but we can’t. You should be talking to your doctor and getting their advice on how to move forward if they’ve identified a mosaic embryo.

Here’s what we can do: Tell you what you might need to consider when considering the risks of transferring a mosaic embryo.

Is it a "high level" or "low level" mosaic embryo?

There’s no exact math here, but if most of the cells are abnormal, your mosaic embryo is likely to be called high level. If there are fewer abnormal cells, it will likely be considered low level.

Should I hold out for a "normal" embryo?

The word “normal” is pretty complicated, isn’t it? No one wants to be called “abnormal,” and an “abnormal” embryo is not ideal. But there’s a whole lot of nuance in embryo descriptions, according to Northwestern’s Jungheim who says to take those descriptors with a grain of salt…and have a long talk with your doctor. “It is important to recognize that even with an embryo that has a ‘normal’ result, the chance of a live birth is still not 100 percent,” she says. This is where finding a reproductive endocrinologist you trust makes all the difference.

Can the embryo "self correct"?

You might have heard this from some other folks in your fertility community. But “self-correcting” mosaic embryos are a bit controversial in the medical community. “More than likely, normal cells have a growth advantage over abnormal cells,” Flanagan says.

We’ll put it in math terms. Say there are 10 “abnormal” cells and 10 “normal” cells. If the 10 normal cells grow and divide more successfully than the abnormal ones, over time there could be 100 normal cells but just 15 abnormal ones. That “could” be a self-correction.

Then again, sometimes the abnormal cells your reproductive endocrinologist found when they were testing the TE cells were only in the TE. Genetic testing is done on TE cells — remember, those are the ones responsible for creating the placenta — because doctors don’t know what would happen if they biopsied the ICM cells (the ones that actually make a baby!). And it turns out sometimes the ICM cells are just fine…even if the TE cells showed mosaicism.

“The TE is prone to cells with chromosomal issues, while the ICM does not tolerate abnormal chromosomes well,” Flanagan explains. “Therefore, an embryo with a very ‘abnormal’ TE can still result in a healthy pregnancy and baby if there are few or no abnormal cells in the ICM.” The only way to know? Like everything else, talk to your doctor or find a genetic counselor who can help you through it all.

Putting it all together

The words “mosaic embryo” may not mean the end of the road for your fertility journey. It just means a long talk with your fertility specialist. As Jungheim says, it all comes down to the specific chromosomes involved and the proportion of cells. Good luck!