Understanding blastocyst quality and it's role in IVF success

Why the Blastocyst Stage Matters

Understanding what blastocysts are and how their quality is assessed can make the IVF journey feel less daunting and help you follow each step of treatment with more clarity.

Reaching the blastocyst stage is graduating from the first, most important week of embryo development, and the embryo has grown strong enough for implantation.

"A blastocyst is the final stage of embryo development, ready for implantation. It develops about five or six days after fertilisation. An embryo transfer of a blastocyst has a higher potential to implant than a cleavage-stage embryo, which is about 2–3 days old with just a few cells,” Dr Liow explains.

At this stage, the embryo has many cells and begins forming distinct parts:

• Inner Cell Mass (ICM) – will develop into the baby
• Trophectoderm (TE) – will form the placenta
• Blastocoel Cavity – a fluid-filled space that supports growth

Not all embryos reach the blastocyst stage, and that’s completely normal. In the first few days after fertilisation, embryos undergo rapid cell division. Some stop developing if they lack the cellular structure or energy to continue.

Those that reach the blastocyst stage are usually stronger and have a higher chance of implanting successfully. However, the IVF success rate still depends on the quality of the embryos.

“A good-grade blastocyst has about a 50–70% chance of a successful IVF outcome; a lower-grade blastocyst has less than 50%,” Dr Liow says. “That said, a higher grade does not guarantee pregnancy or a healthy baby.”

Genetics is another consideration.

“For example, a euploid embryo, one with the correct 46 chromosomes, has a good chance of a healthy pregnancy. An aneuploid embryo, which has extra or missing chromosomes, may fail to implant or lead to pregnancy loss,” he explains.

Does a lower-grade blastocyst affect pregnancy outcome?

“Not necessarily,” Dr Liow says. “They can still lead to healthy babies.”

Transferring embryos at the blastocyst stage also matches the natural timing in the body, when an embryo would normally enter the uterus. This allows doctors to transfer just one blastocyst in many cases, reducing the need for multiple embryos and lowering the risk of multiple pregnancies.

Embryos that do not reach the blastocyst stage are usually unsuitable for transfer. Your clinic care team will advise if they are kept under observation for an extra day, but in most cases, they are respectfully discarded.

Embryo Grading

It is the main tool embryologists use to evaluate blastocysts’ quality in the laboratory. This system assesses the embryo's appearance under a microscope, focusing on the size, shape, and uniformity of cells. At Virtus Fertility Centre, we use a global standard called the Gardner grading scale that combines a number and two letters:

• The number (usually from 1 to 6) indicates the stage of blastocyst expansion, with higher numbers meaning the blastocyst is more developed and ready for implantation.
• The first letter grades the Inner Cell Mass (ICM) quality, or how well the cells that will become the fetus are formed.
• The second letter grades the Trophectoderm (TE) quality, as in how well the cells will develop into the placenta.

Each letter grade can be A, B, or C:

• A means the cells are tightly packed, uniform, and appear healthy.
• B indicates some minor irregularities, but generally good quality.
• C suggests fewer cells or uneven shapes, which may indicate lower quality.

For example, a blastocyst graded 5AA is well-expanded (stage 5) with high-quality inner cell mass and trophectoderm. Such embryos typically have a better chance of implantation.

Embryologists use this system to choose the embryos most likely to implant and grow into a healthy pregnancy. It’s a bit like giving each blastocyst a “report card” before transfer, helping patients and doctors make the best possible decisions.

“Patients are generally concerned as lower-grade embryos have lower potential to generate a healthy pregnancy. However, healthy babies have been born from lower-grade embryos. They should remember that grading is only one part of the story. The uterus environment, genetics, and overall health are just as important,” he says.

Factors affecting blastocyst development

Blastocyst development is susceptible to laboratory conditions. High-quality culture media provide essential nutrients, while stable incubator temperature and pH prevent stress on embryos. Clean air and strict sterility reduce toxins and contamination risks. Skilled embryologists ensure gentle handling during procedures, and controlled light and low-oxygen environments minimize DNA damage, all of which increase the likelihood of healthy blastocyst formation.

While lab quality can directly impact embryos reaching the blastocyst stage, the most important is egg quality, which is closely linked to the woman’s age and overall reproductive health. Sperm quality also plays a role, as healthy DNA from both egg and sperm is needed for normal development.

Some embryos stop developing naturally if they have genetic abnormalities or lack the energy to continue dividing. This is part of nature’s way of selecting the embryos with the best chance of resulting in a healthy pregnancy.

When deciding how likely an embryo is to lead to a healthy pregnancy, the grade (its appearance under the microscope) is only one part of the picture. Other things, especially the genetic makeup of the embryo, are just as important. This is where Preimplantation Genetic Testing for Aneuploidy (PGT-A) can help.

“PGT-A is recommended in cases of repeated pregnancy loss, failed implantation, and, possibly, in advanced maternal age, as in more than 40 years old. Important to note that PGT-A does not guarantee a healthy pregnancy and live birth. It is a tool to identify an embryo that is euploid and has the potential to generate a pregnancy,” he adds.

Key Points to Remember

Blastocyst quality grading and genetic testing give your fertility care team more insights, but nothing can predict the outcome with certainty.

“Embryo quality is only a part of a puzzle to a successful IVF programme. Many healthy babies have been born from embryos that looked less than ideal in the lab, though at a low but non-zero rate,” Dr Liow says. “Most importantly, stay healthy and have faith in oneself that good things will come your way.”