How to IVF: Embryo Development for Beginners

Day three and day five embryos. TESA. Blastocyst. ICSI. Assisted hatching.

IVF terminology can sound like another language, especially when it comes to embryo development. Embryology – the creation of embryos from oocytes and sperm – is when the magic really starts to happen. Unfortunately, understanding the science of embryo creation can be complex and anxiety-inducing for hopeful parents. 

Our goal at Love & Kindness Surrogacy is to help demystify the IVF and surrogacy processes for you. Here are things that you should know about how embryos develop and what it can mean for your outcomes. 

Timeline for embryo development

You’ve got your eggs retrieved (or dethawed). You’ve got your sperm (also collected or dethawed). Now it’s time to create embryos. Your clinic’s embryologists are the medical professionals in charge of this process. 

Did you know? Embryologists have at least a master’s degree in reproductive or clinical science, with some also having a medical degree. Further training varies in different labs based on lab management.

Fertilization

This process is called fertilization and it takes place in the embryology lab. An embryologist uses a process known as intracytoplasmic sperm injection (ICSI) to inject a single spermatozoon into an egg. 

Note: Some labs may choose not to do ICSI depending on a number of factors. There are some risks associated with the procedure. However, it has become the industry standard in recent years because of its overall higher success rate. 

Evaluation

After fertilization, the embryos are cultured in incubators that are designed to mimic the environment of the human fallopian tube, using a careful balance of carbon dioxide, oxygen, and key nutrients to support growth. 

Note: Standard practices vary from clinic to clinic. This information is intended to provide a general overview of the process for informational purposes. For specifics on how your clinic operates, please speak with your client care specialist. 

Day 1: You’ll receive your fertilization report. At this stage, embryologists hope to see healthy developing embryos show two pronuclei in their center. 

What you’ll hear from your clinic: Almost all clinics provide a report following fertilization. This report will let you know how many eggs were fertilized

Day 2: As embryos continue to develop, they should have divided into 2-4 cells. 

What you’ll hear from your clinic: The majority of clinics don’t call on the second day, as it’s important to let the embryos grow undisturbed. 

Day 3: By day 3, embryos ideally have divided into 6-8 cells. Up until day 3, maternal DNA drives development, but at some point on day 3, paternal DNA begins its contributions to embryo growth. If there is male factor infertility, this is often where it begins to affect embryo quality. 

Depending on a variety of factors – quantity and quality of embryos, the progress of development, patient history, and clinic practices – some doctors may opt to transfer on day 3. However, most clinics prefer to wait until day 5 of development to transfer. 

What you’ll hear from your clinic: Practices vary from clinic to clinic. Some may deliver day 3 reports while others wait until transfer on day 5 to give a final report. 

Day 4: By this time, a healthy growing embryo usually has 8-10 cells. Around this time, the cells begin to compact, or fuse together, which means the embryo has reached the morula stage!

What you’ll hear from your clinic: Practices vary from clinic to clinic, but no reports on day 4 are common. However, your clinic will likely contact you on this day to coordinate a transfer time on day 5.

Day 5: By this time, the embryo contains between 70-100 cells and is highly developed. Not all embryos grow to the blastocyst stage; rates vary, but approximately only ⅓ of embryos make it to this point.

What you’ll hear from your clinic: If your clinic does day 5 transfers, you’ll receive a report of how many embryos have grown to the blastocyst stage. If you have multiple embryos, you find out how many can be cryopreserved for future transfers. 

Grading

Grading blastocysts: How blastocysts (commonly referred to as blasts) are graded depends on the clinic; some clinics use scales of 1-6, others use Good/Fair/Poor, and others use a combination of letters and numbers. It can get confusing! Each clinic should be able to provide you with their metrics. Here is an overview of what is typically evaluated.

Expansion

The embryo expansion looks at the size of a blastocyst. Early blastocysts have fewer cells and a poorly defined inner cell mass and trophectoderm (see below) whereas blastocysts that are further along in development have a greater number of cells and may even show the embryo hatching or hatched out of the zona pellucida (shell).

Inner cell mass 

The inner cell mass represents the collection of cells that are destined to differentiate into the fetus/baby. 

Trophectoderm

The trophectoderm represents the collection of cells that will become the placenta and are responsible for the initial invasion and implantation of the embryo into the uterine lining. 

What to remember

Embryo grading can be a cause of a great deal of worry before a transfer, but it’s important to remember that how an embryo is graded doesn’t always reflect its potential. Many embryos receive low grades by the embryologist only to become healthy babies. 

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