What Is A Mock Cycle and What Role Does It Play In IVF Treatment?

There’s a lot of terminology floating around, and even those who have gone through IVF before might encounter new lingo. Today we’re demystifying one of these: the mock cycle.  

What is in vitro fertilization (IVF)? 

In vitro fertilization (IVF) is one type of assisted reproductive technology (ART). It involves retrieving viable eggs that are fertilized outside the body and then transferred to the lining of the uterus for implantation. An IVF cycle has five main steps: 

1. Pre-cycle fertility testing for women:  
   Before treatment can begin, an extensive series of tests are performed to better understand how infertility is impacting your ability to conceive. 

2. Stimulation of the ovaries to produce several eggs during one menstrual cycle:  
   A regimen of injectable medications is administered over a two-week period to induce the production of multiple eggs in one cycle. 

3. Retrieval of the eggs from each ovary:  
   An outpatient surgical procedure is performed to extract the mature eggs from your ovaries. 

4. Fertilization of the eggs in a laboratory by either conventional fertilization or intracytoplasmic sperm injection (ICSI):  
   Your eggs will be introduced to the sperm sample and then watched over by an embryologist for about five days. 

5. Embryo implantation into the uterus:  
   Once your embryos have matured enough, there is another outpatient procedure to transfer them to your uterus for implantation. 

What is a mock cycle? 

Not to be confused with a mock embryo transfer, a mock cycle in IVF treatment is a procedure that replicates the physiological conditions that you’ll experience during an IVF cycle so that endometrial testing can be performed. Unlike a full IVF cycle, a mock cycle does not include an embryo transfer. Instead, an endometrial biopsy is performed on the day when the transfer would ordinarily take place. The goal of this procedure is to get a better understanding of your window of implantation (WOI) by analyzing the receptivity status of your endometrium on the day when you were predicted to be most receptive. Mock cycles can be medicated or unmedicated, depending on the specific treatment protocol your fertility specialist has designed to fit your fertility needs. 

What is your window of implantation (WOI)? 

The window of implantation is the two- to three-day window during a menstrual cycle when the endometrium is the most receptive to embryo implantation. On average, the WOI occurs on days 19 to 21, but for as many as 30% of infertility patients it can occur at a non-standard time or be shorter than average. If you have a non-standard WOI, it can mean that your embryo transfer procedure might not be scheduled at the optimal time. 

The mock cycle timeline 

For IVF veterans, the following timeline may look familiar. That’s because it is the exact same timeline as in vitro fertilization with frozen embryo transfer (FET-IVF). As mentioned above, the key difference is that there will be no embryo transfer at the end of the cycle. The following is a breakdown of each step and when in the cycle they occur (although it should be noted that your personal mock cycle timeline will reflect your fertility needs and goals): 

Once you’ve had the biopsy, you should expect to start your period within a week or two. Your fertility specialist will advise you on when to expect your results back. Wait time may vary based on the size and caseload of the testing lab. 

Why are mock cycles performed? 

The simplest answer is that mock cycles are performed when your fertility specialist feels they need more accurate information about your endometrial receptivity in order to optimize the timing of your embryo transfer. Below, you’ll find some of the most common reasons your doctor may recommend having a mock cycle. 

Your previous IVF cycle failed 

It is very common to undergo more diagnostic testing after a failed IVF treatment cycle in order to identify any underlying issues that prevented successful pregnancy. As endometrial receptivity plays a key role in the embryo implantation process, recreating the physiological conditions of an IVF cycle will better inform your next embryo transfer procedure. 

You’ve experienced repeated implantation failure (RIF) 

Repeated implantation failure (RIF) can also be described as multiple miscarriages. This occurs when the endometrium rejects an egg or halts the development of a fetus. Research has shown that having an accurate picture of a patient’s receptivity status can greatly improve IVF success rates for those who experience RIF. 

You have a limited number of embryos 

For prospective parents who only have a set number of embryos, it can be especially important to ensure each embryo transferred has the best chance to implant. Undergoing a mock cycle before doing FET-IVF will help determine the most optimal timing for your embryo transfer. 

Innovations in implantation optimization 

Every endometrial receptivity test currently on the market, including Inti Lab’s MIRA™, requires a mock cycle and endometrial biopsy. However, we’ve developed a new test, ora™, that only requires a blood sample to determine endometrial receptivity through the use of blood-based microRNA biomarkers. This blood sample still needs to be taken on the same day a biopsy would occur, but it can also be taken on the day of an embryo transfer. This means that ora™ has the potential to eliminate the need for a mock cycle altogether. For more information about microRNA and the science behind endometrial receptivity testing, you can read our explainer here.