The implantation process
Early Pregnancy Signs & Symptoms Two Week Wait

The Mystery of the Implantation Process Revealed

Referred to as the "black box" of an IVF cycle, knowledge around the implantation process is severely limited. Here's what you should know.

The implantation process is the last – and most critical – step of an IVF cycle, but it’s also the step that we know the least about as it is both unobservable and unassisted. Since our last embryo transfer, I’ve spent the past few days in two-week-wait hell thinking a lot about the process of implantation and this mysterious journey that our embryo must make alone. Read below to discover the crucial steps of the implantation process, the timing of implantation, and the very early signs of implantation success.

The Implantation Process – A Solo Journey

During a typical cycle of IVF, all variables of conception are closely monitored and/or assisted, including egg selection, sperm selection, and preparation of the uterus. But once it comes time to transfer the embryo into the uterus, all “training wheels” come off and the embryo must complete the implantation process on its own.

The implantation process is when the embryo attaches itself to the inner lining of the uterus, or “endometrium.” It sounds simple enough, but the fragile embryo’s successful attachment to the endometrium is quite nuanced and complex. Moreover, there is no scientific mechanism that allows doctors to assist or observe this complex process. For this reason, implantation is often referred to as the “black box” of conception, as doctors can’t monitor the embryo until after implantation has occurred and the embryo – now called a gestational sac – is visible via ultrasound, which is approximately 5-weeks of pregnancy.

Implantation Timing – The 3 Stages of Implantation

The process of implantation is initiated when the embryo comes into contact with the uterine wall and can be broken down into three stages: (source)

  • Apposition – The very first, albeit loose, connection between the blastocyst and the endometrium.
  • Adhesion – Adhesion is a much stronger attachment to the endometrium than the loose apposition. The trophoblasts adhere by penetrating the endometrium, with protrusions of trophoblast cells.
  • Invasion – Invasion is an even further establishment of the blastocyst in the endometrium. The protrusions of trophoblast cells that adhere into the endometrium continue to proliferate and penetrate into the endometrium. Finally, the whole embryo is embedded in the endometrium.
Implantation Process


 “Embryo implantation is the result of a well-orchestrated sequence of events […], in which various factors come into play one after the other, yet remaining in close collaboration.” – source

For more visual learners, Kahn Academy, a free online learning center, provides a fantastic video overview of the implantation process.

Early Implantation Signs

Very early signs of implantation are implantation cramping or implantation bleeding, but keep in mind that these do not happen for every woman and are often unreliable or inconsistent. The first – and only – sign of successful implantation is the rising levels of human chorionic gonadotropin (hCG) in a woman’s body. Referred to as the “pregnancy hormone,” hCG can be detected in either the blood or the urine only after implantation has occurred. 

The first – and only – sign of successful implantation is the rising levels of human chorionic gonadotropin (hCG) in a woman’s body.

A home pregnancy test can detect the presence of hCG in urine once hCG has reached a certain threshold. In a study reported by CBS News, Consumer Reports reveals that First Response Early Results (FRER) home pregnancy test can detect hCG levels at concentrations as low as 6.5 mIU/ml (thousandths of an International Unit per milliliter) in urine. The report states: “That’s almost sensitive enough to detect any pregnancy soon after implantation.”

This makes the FRER home pregnancy test the most sensitive test on the market. Just keep in mind that this sensitivity comes with a price tag, as FRERs are also the most expensive tests on the market.

For those that are too curious to wait for the home pregnancy test, BabyMed offers a handy implantation calculator and an implantation quiz you can try below!

Implantation Calculator

Implantation Quiz

Timing of Implantation

Implantation can happen as early as six days and as late as 12 days after fertilization/ovulation (source). IVF patients can also consult the below chart that outlines what happens after an embryo transfer. According to this chart, implantation is complete five days after a “day 5” embryo transfer, and seven days after a “day 3” transfer.

What Happens After IVF Embryo Transfer

Implantation Failure

When implantation fails to occur, it is most often because the egg stopped developing before it was able to implant itself. This is called “oocyte arrest” and was often thought to be on account of a “bad egg.” However, new technologies that can now test for egg quality – like PGD and PGS testing – reveal that what were often deemed “bad eggs” successfully implant and what were deemed “good eggs” fail to implant. Therefore, more research is beginning to focus on the complex implantation process and what actually happens (or fails to happen) during this crucial, but unobservable time.


The least controlled step of the IVF process, implantation is truly a mysterious phenomenon that scientists are still learning much about. However, understanding the basic steps of the implantation process, the early signs of its success, and when to accurately test to see if implantation occurred  can make this process way less stressful!

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