What Is IVF Blastocyst Culture? – Success Rates & Timeline

In recent years, blastocyst culture is becoming increasingly popular in almost every In Vitro Fertilization (IVF) procedure due to better implantation rates with blastocyst embryos (day 5-6 of development) than with embryo transfer at earlier stages.

One of the multiple advantages it offers is that is allows for a better selection of embryos. To this end, blastocysts are cultured based on their morphology and grade of expansion, that is, their stage of development: early, cavitating, expanded, hatching, etc.

What is blastocyst culture?

Blastocyst or extended embryo culture involves keeping the embryos at the laboratory until day 5 or 6 of development, when they reach a stage that is known as blastocyst, hence the name.

Blastocysts are made of of two clearly identifiable parts:

Trophoblast
Composed of the cells of the blastocyst that are in direct contact with the uterine lining (endometrium). This structure will turn into the placenta eventually. The outer layer is protected by the zona pellucida (ZP).
Inner cell mass
Located within the trophoblast. It develops into the fetus.

When an embryo attaches naturally to the uterine lining, it is at blastocyst stage. If embryo development occurs artificially at an IVF laboratory, it should be cultured in optimal conditions in order for it to reach the blastocyst stage and still be a high-quality embryo, with implantation potential.

Types of embryo culture

Extended embryo culture can be done in two different culture media, as one shall see in the sections below:

Embryo culture using sequential media

In order to allow embryos make it to blastocyst stage, they are cultured in two media of different chemical composition to adapt to the nutritional requirements of embryos at each phase of their development.

Embryos are cultured in the first medium from the zygote (day 1) stage until day 3. The second media will be used from day 3 until blastocyst stage (days 5-6 of development).

Embryo culture using global media

In this case, embryos are cultured in a single type of medium that contains all the necessary nutrients embryos need from day 1 of development until the transfer or vitrification day on day 5-6, when they reach the blastocyst stage.

Blastocyst grading

To predict the implantation potential of blastocyst embryos, one of the most commonly used methods nowadays is blastocyst grading. Thanks to it, we can classify embryos based on their quality.

Embryo grading is done by evaluating the degree of expansion, their inner cell mass (ICM), and the trophoblast. For these parts to develop as they should, the blastocoel must be present—a fluid-filled cavity that forms in blastocysts.

According to these three parameters, the traditional method used to classify embryos uses a figure (1-5) and two letters (A-D), based on the degree of expansion, the quality of the ICM, and the trophoblast, respectively.

For example, following the criteria that we will explain below, an expanded blastocyst of good quality would be classified as 3AA.

Degree of expansion

The numerical score of blastocyst embryos is established using the following numerical score system:

Grade 1 or early blastocyst
The blastocoel starts to be visible.
Grade 2 or cavitating blastocyst
Both the trophoblast and the ICM are distinguishable.
Grade 3 or expanded blastocyst
The blastocyst has increased in size. The layer that covers it, the ZP, is thinner now.
Grade 4 or hatching blastocyst
The blastocyst is starting to hatch out of the ZP.
Grade 5 or hatched blastocyst
The blastocyst is already completely hatched.

Inner cell mass (ICM)

When it comes to evaluating the ICM, the embryologist takes into account its size, degree of compaction, and its appearance:

A score
Multiple compacted cells.
B score
Multiple non-compacted cells.
C score
Only a few cells.
D score
Cells with signs of degeneration.

Trophoblast

As regards the trophoblast, which should be formed of a single layer, the parameter examined is its appearance:

A score
Homogeneous, cohesive, and with multiple cells.
B score
Homogeneous and with a lower number of cells.
C score
With a reduced number of cells.
D score
Cells with signs of degeneration.

Uses

Blastocyst culture is not used in all cases, as it depends on various factors, but basically on the number of embryos available to transfer. Blastocyst IVF is recommended in the following situations:

  • IVF cycles in which the number of high-quality embryos is high on day 3 of development.
  • Cases of repeated implantation failure.
  • Patients who undergo Preimplantation Genetic Diagnosis (PGD).
  • Patients of advanced maternal age.
  • Cases in which multiple pregnancy is contraindicated.
  • To optimize a cycle of vitrification, since waiting up until the embryo reaches the blastocyst stage to cryopreserve the embryos allows us to store only those with the best quality.

Pros and cons

Extended culture increases the overall success rates of IVF cycles because:

  • It increases the implantation rates, as it allows for an improved embryo selection process.
  • It diminishes the risk of multiple births, as it allows for Single Embryo Transfers (SETs) without affecting the pregnancy success rates.
  • It enhances the synchronization between the embryo and the endometrial lining.

Nonetheless, in spite of the advantages that blastocyst embryo transfers offer, it is not indicated for all kinds of patients due to the cons associated:

  • Many embryos arrest before even making it to blastocyst stage. Thus, you would be putting yourself at risk of having no embryos available on the transfer day.
  • Not all IVF laboratories reach good outcomes with extended embryo culture.
  • It is necessary to have a significant number of eggs/embryos available to make sure that a certain number of embryos will be available on the transfer day.

FAQs from users

When does the embryo implant after blastocyst transfer?

It takes around 10-11 days approximately for a blastocyst embryo to implant after a blastocyst embryo transfer.

Can poor quality embryos implant as well?

Absolutely yes. The chances for pregnancy with poor quality blastocysts are lot, but they exist. Pregnancy is possible as well.

What percentage of embryos make it to day 5 blastocyst?

It is estimated that only 20-50% of day two embryos can continue developing in vitro to day 5, irrespective of how perfect the in vitro culture system is.

Suggested for you

If you are interested in learning more about embryo culture, a process that takes place at the IVF lab, we recommend that you have a look at this: Embryo Culture Media for Human IVF.

Also, you can find more details about the difference between day 3 and day 5 embryo transfer, visit the following post: Day 5 vs. Day 3 Embryo Transfer – What Are the Pros & Cons?

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