How endometrial receptivity impacts fertility
An interview with Dr. Chan, Scientific Director at Markham Fertility Centre
On EP11 of My Fertility Podcast we host guest Dr. Chan, the Scientific Director of Markham Fertility and she is a Reproductive Endocrinologist, minimally invasive surgeon, and an Assistant Professor in the Department of Obstetrics and Gynaecology and the Department of Lab Medicine and Pathology at the University of Toronto.
Dr. Chan is an expert on many things related to fertility, however, there is one particular area that she is very passionate about and that is the endometrium. We will answer some of the most common questions related to the endometrium and its role in fertility:
- What is the endometrium and how does it affect fertility?
- What is endometrial receptivity and when does it happen during the menstrual cycle?
- How do we create endometrial receptivity in a frozen embryo transfer (FET) cycle?
- How do you test endometrial receptivity?
- What is the protocol for an embryo transfer in somebody with endometriosis?
- Are ERA tests recommended?
- What does the future hold for endometrial receptivity assessment?
Why did you choose to research the endometrium in particular?
Dr. Chan: I knew I wanted to be a fertility doctor from very early on. In medical school, I learned about what IVF was. I don’t think I had a great awareness of what IVF was prior to medical school, but I do remember learning about it at that time.
I learned about the pioneers of IVF, Dr. Robert Edwards, Dr. Patrick Steptoe, and their nurse, Jean Purdy. These people were visionaries! In the 1970s, they devised a way of combining oocytes or eggs and sperm to make embryos and putting those embryos back into the uterus to achieve pregnancy and they were really innovative at the time. People thought they were crazy. This was the 70s and they got brutal hate mail accusing them of perverting nature. But these visionaries didn’t care; they kept their noses down, they kept working because they knew that the technology would make such a difference to humanity that they just kept going. And here we are, almost 50 years later and almost 10 million IVF babies later. And looking back, it’s thanks to them and their innovative spirit that we’re even here.
So when I learned about how cool IVF was and when I learned about what the pioneers of IVF had to go through with their innovative spirit and their commitment to science, I wanted in. I wanted to do exactly that for my career. I wanted to be a fertility doctor. I wanted to be a researcher and a scientist.
Robert Edwards, the father of IVF, wrote a paper in 2006 entitled Human Implantation, the Last Barrier in Assisted Reproductive Technology. He spoke about all the different things that were required for implantation to occur. And a lot of it is up to the embryo, the embryo’s health, the embryo’s genetics, and the embryo’s energy levels.
But we know that it’s not just about the embryo. So even at top clinics like MFC, even with genetically screened, top-quality embryos, the pregnancy rate we know is about 70% per embryo transfer. So what is happening to those 30% of good embryos that we’re putting into the uterus that aren’t implanting?
So really it’s not only a good embryo that’s needed, but what’s equally important is a healthy, functional, viable, lush, receptive endometrium. At that time, not many people were studying this black box (that’s what I call it, the endometrial black box). You can call me a contrarian but I wanted to study what other people were neglecting. And so that’s what I did. I did a Master of Science degree in my residency. I traveled to the University of California in San Francisco for a visiting fellowship to learn how to culture or grow endometrial cells in the laboratory.
Tell us more about the uterus and in particular, what exactly is the endometrium?
Dr. Chan: Think about the uterus as a squash. I like to use this analogy. There are three main layers of the uterus. The outside layer of the squash, or the peel, would be what’s called the serosal layer, or serosa of the uterus. The middle layer of the squash, which would be the flesh of the squash, would be the myometrium, which is mostly muscle. And then the inner layer of the squash, the part that has the seeds, is the inner lining in the uterus called the endometrium. So from the outside in, you have serosa, myometrium, then endometrium. The endometrium is the most sacred part. That is the part where all the action happens in terms of implantation.
The endometrium is the part we, as reproductive endocrinologists, probably care about the most when we talk about fertility and assisted reproduction.
I want to talk a little bit about what the endometrium does over the course of the menstrual cycle. It’s beautifully predictable and dynamic throughout the cycle. Right after the period, under the influence of estrogen being produced by the ovary, the endometrium thickens, also known as proliferation. This phase of the menstrual cycle is known as the proliferative phase. Midway through the cycle ovulation happens in the ovary. After ovulation, progesterone is produced by the ovary, and progesterone causes the endometrium to stop thickening and become what’s called secretory. This is when the endometrial cells start to produce a host of different factors that are meant to (hopefully) facilitate embryo implantation.
If embryo implantation occurs, then the endometrium stops thickening and continues to secrete these factors to facilitate implantation. However, if pregnancy does not occur, that endometrium breaks down and sheds as another period and the whole cycle repeats.
When we’re talking about embryo implantation in a naturally conceived pregnancy, the embryo drops into the uterine cavity by coming down the fallopian tube and into the endometrium. In the case of an embryo transfer in IVF, an REI will insert the embryo through the cervix from below into the uterine cavity. It ends up in the same place. So at some point, we hope that the embryo and the endometrium have a chance of meeting.
For implantation to occur, the embryo and the endometrium have to communicate. There has to be a dialogue between the two, and we think a lot of that happens on a molecular level. Three phases need to happen:
Phase I. The embryo has to oppose or cozy up to the endometrium
Phase II. Next, the embryo has to attach but the endometrium has to allow it to attach, which means they bind together in a very tight and fixed way
Phase III. Finally, that embryo has to invade and implant into the endometrium, so the endometrium has to permit the embryo to invade or grow into itself
What is endometrial receptivity and when does it happen during the menstrual cycle?
Dr. Chan: Endometrial receptivity is the ability of the uterus, or endometrium, to facilitate embryo implantation. The endometrium probably isn’t just a passive thing that allows every embryo to implant. It’s a very intentional state of the endometrium to be able to allow that embryo to oppose, attach, and invade. In fact, we know that this is a very time-limited situation.
The endometrium is not always receptive to the embryo. In most of the menstrual cycle, it is definitely not receptive. You can take the most beautiful, genetically tested, embryo in the world and you can put it into a proliferative phase lining prior to ovulation and you will not get a pregnancy. At that time of the cycle, the uterus is not receptive so no pregnancy will occur. So when is the endometrium receptive? There have been many classical research studies on this and it seems to be that the natural window of implantation starts approximately five days after ovulation and lasts up to five days.
How do we create this endometrial receptivity in a frozen embryo transfer (FET) cycle?
Dr. Chan: People are freezing embryos these days for various indications. Sometimes people aren’t ready to do their embryo transfer yet, sometimes they’re doing pre-implantation genetic testing, or PGT, and that necessitates having to freeze the embryo for a later transfer.
I would say at this point frozen embryo transfers are more common than fresh embryo transfers. When we’re trying to do a frozen embryo transfer, we’re trying to mimic or recreate the natural menstrual cycle and the natural secretory phase.
In a medicated frozen embryo transfer cycle, we prescribe the patient estrogen tablets and/or patches for the purpose of trying to thicken the endometrium to “copy” the proliferative phase. This often lasts about two weeks. This is followed by an ultrasound, and if the lining looks adequate, then we will start progesterone. On the sixth day, usually, of progesterone administration, we will thaw an embryo and transfer that embryo, and that is meant to recreate the window of receptivity.
There’s also another way to do embryo transfers called natural frozen embryo transfers where we don’t give estrogen, and we monitor via cycle monitoring of a natural cycle to watch for ovulation and then transfer the embryo six days after the luteinizing hormone (LH) surge.
In both medicated FET and natural FET protocols, we’re trying to recreate that window of receptivity of the uterus.
How do you test endometrial receptivity?
Dr. Chan: When assessing for endometrial receptivity, the standard test is ultrasound. Every fertility doctor uses ultrasound as their main diagnostic tool. We’re very comfortable with it. It has become the standard of care for assessing the endometrium or lining prior to embryo transfer in IVF. For the past 30 years, transvaginal ultrasound measurement of the endometrium has been the mainstay of determining if the endometrium is ready before embryo transfer. So when do we measure the thickness of the endometrium with ultrasound? It is measured at the end of the proliferative phase. So let’s say you have a patient who’s about to have a frozen embryo transfer, and let’s say we’re doing a medicated frozen embryo transfer protocol. This person has been on estrogen tablets and/or patch for about two weeks and this is when we usually do an ultrasound to gauge the thickness of the endometrium. We measure the endometrium and then we use that endometrial thickness measurement as a predictor of whether the endometrium is ready for implantation or not.
The most common question is, how thick is thick enough? There was a publication by one of my former colleagues, Dr. Kimberly Liu which looked at just that. It was a fantastic study looking at over 40,000 data points from fresh and frozen embryo transfer cycles using Canadian Assisted Reproductive Technology Registry data and what the authors found is that for frozen embryo transfers, an endometrial thickness of 7 millimeters and above is optimal for prediction of implantation. For every millimeter under 7 millimeters, the pregnancy rate was reduced. So from this study we can conclude that 7 millimeters is the optimal threshold.
But, we all know that some patients can’t get to a lining thickness of 7 millimeters, so it is important to note in this study that the pregnancy rate never went to zero at any endometrial thickness. They even reported pregnancies at 4 millimeters of endometrial thickness, which traditionally was too thin to transfer.
I don’t think that endometrial thickness is the be-all and end-all of embryo transfers. Yes, 7 millimeters and greater is ideal and is the target, but some people don’t get to that and pregnancy can still occur. Therefore, endometrial thickness is an “okay” predictor of implantation and endometrial readiness, but it’s not everything.
Many people have also tried to look at the pattern of the endometrium at the end of the proliferative phase. Traditionally, we like to see what’s called a trilaminar or triple line pattern of the endometrium and if you look at ultrasound images, you can see this quite clearly sometimes. What that means is you see three stripes or three lines through the endometrium. It is thought to be a more favorable pattern that predicts implantation. But, if you look at the research studies, it hasn’t really panned out that it’s an accurate predictor of implantation. Most fertility doctors do not use this as an indicator.About 15 years ago, many researchers, including myself, started to look at the endometrium and to see if there were any molecular factors that might be able to predict implantation. We wanted to go beyond just looking at the lining on ultrasound, by looking at genes and looking at DNA expression. There was a lot of excitement when endometrial receptivity analysis or ERA was released. It was really a pioneer in the analysis of the endometrium. It was the first product to come to market that was a molecular test for endometrial receptivity.
How the ERA test works
The ERA is meant to help you determine the best time to transfer a frozen embryo. Let’s say you have a patient for whom you’re trying to do a medicated frozen embryo transfer. You are trying to determine the best time to put the embryo in. You would put this patient through what’s called a mock cycle, or practice cycle and give that person estrogen for approximately two weeks, and if the lining got thick enough, you would then give them progesterone. On the sixth day of progesterone, usually when we would do an embryo transfer, we do an endometrial biopsy instead. An endometrial biopsy is a test where we insert a tube into the uterus and extract some uterine lining cells. It can be painful and crampy but its very quick. One we complete the biopsy we can send those cells for ERA testing. This is not covered by any provincial health plan and is a private pay procedure. The ERA test will analyze about 250 genes that have been shown to be predictive of the endometrium’s ability to implant and from the pattern of these genes, the test would tell you if the uterus or the endometrium is receptive or non-receptive at that particular time (in this example the sixth day of progesterone administration).
The idea is that if the endometrium is receptive, then its a good time to do your embryo transfer, and we would transfer on the sixth day of progesterone treatment in a subsequent cycle. If it’s not receptive, it would either be what’s called pre- or post- receptive. The test would suggest that you adjust your day of transfer one day later than the biopsy or one day earlier. The test also claims that the window of implantation is 12 to 24 hours, which is much shorter than earlier studies suggested.
What is the protocol for an embryo transfer in somebody with endometriosis? Do you recommend an ERA?
Learn more about endometriosis in an interview with Dr. Zeni
Dr. Chan: Endometriosis is a complex and heterogeneous disease. Heterogeneous meaning that there’s a whole bunch of different ways somebody with endometriosis can present. Some present with pain, some present with no pain. Some present with infertility, some are very fertile. It’s not an absolute and endometriosis does not define you and that’s part of the difficulty with endometriosis research in IVF.
First and foremost, not everybody with endometriosis has infertility or implantation failure. If you look at the data comparing patients with endometriosis versus patients with no endometriosis, both gorups have very similar implantation rates when you’re putting in euploid, genetically screened embryos.
You can’t cure endometriosis – that’s another important message. However, many people have tried to give medications to make endometriosis dormant, such as leuprolide, which puts people in a temporary menopause, and then proceed with a frozen embryo transfer to see if they can improve pregnancy rates.
I think my answer to the question is that I don’t treat people with endometriosis differently from an FET protocol standpoint. However, if there’s a patient with endometriosis that has failed multiple embryo transfers, sometimes I will try pretreating them with endometriosis medications, because there’s not much else we can offer. But, it’s not my go-to because it hasn’t been proven to increase implantation rates.
What does the future hold for endometrial receptivity assessment?
Dr. Chan: I think this is where the industry has to spend some time and attention. Let me be cynical for a moment. 20 years ago, the pregnancy rate per embryo transfer in the world was probably about 20%. Now, in top clinics like MFC, when we genetically screen embryos and put in high quality, genetically tested embryos, the pregnancy rate has improved to about 70% per embryo transfer.
Develop directed therapeutics targeting endometrial receptivity
We have done remarkably well, as a group of scientists and clinicians. But, I’ve noticed, and I think we’ve all noticed, that there’s been a bit of a plateau. This 70% has been the case for the past five to seven years. It hasn’t really gotten appreciably better in the past little while.
We’ve gotten very good at screening embryos, developing embryos, choosing embryos, but we haven’t done anything on the side of the endometrium to directly improve endometrial receptivity and therefore pregnancy rates. The only thing we’ve done as an industry is to shift the day of your transfer sometimes. The next step is to develop directed therapeutics to use the DNA gene expression data that we’re getting from the endometrium (and we’ve got lots of data!) to actually design therapeutics, medications, treatments that will directly go to the uterus and improve that state of endometrial receptivity and improve the ability of the uterus to accept an embryo. In my opinion, this is something that is bound to happen or should happen.
Maximizing the use of AI
The second thing we need to examine is the use of artificial intelligence or AI. The basic premise of AI in diagnostic healthcare is that maybe the human eye and the human brain are not sufficient to detect subtle patterns that might predict pregnancy outcomes. People are using AI and machine learning to look at transvaginal ultrasound images to see if there is something the human eye can’t pick up that might predict implantation or that might predict endometrial receptivity; so that work is being done now. My research group has published data looking at using AI to examine endometrial biopsy samples under a microscope to see if any patterns are can predict endometrial receptivity and implantation.
Uterine fluid aspiration (UFA)
Finally, in the future we will hopefully have noninvasive or less invasive methods of uterine endometrial sampling. This is what I worked on in my 10 years of doing research on the endometrium. The standard of care for all these molecular tests is to do an endometrial biopsy. Biopsies can be painful and they are invasive and they can’t be done in the same cycle as an embryo transfer. For the past ten years I worked on developing a technique called uterine fluid aspiration or UFA, which allowed us to take out a few microliters of fluid from the uterine cavity right before transferring an embryo. We can glean all the same information on a genetic level that you can get from an endometrial biopsy. This technique is out there and it’s been published and I think if somebody really has the appetite to do it, I could imagine a molecular test based on UFA as opposed to a painful, invasive biopsy. That would be a point of care test meaning that you could test the endometrium right before transferring the embryo. This would be the dream – no pain and a test that tells you exactly what is happening right before the embryo goes in!