Blastocysts are embryos in a stage of development that usually begins in the first 5-7 days after fertilization.
In IVF, embryos in the blastocyst stage usually have the highest chances for successful transfer and pregnancy.
If you are creating embryos for IVF, you can read more here on success rates at different stages.
If you are creating embryos for IVF but are not able to become pregnant or sustain a pregnancy, surrogacy may increase your chances of success. Learn how when you contact our specialists.
Read on to learn what a blastocyst is and why blastocyst development is a key marker in IVF success.
What is a Blastocyst? [Definition for Pregnancy]
A blastocyst is an embryo in a specific stage of development that is (or will soon be) ready for implantation.
Because blastocysts are in a stage where pregnancy is likely to occur, it is one of the most common stages for an embryo to be transferred to the uterus in IVF.
Blastocyst [Biology Definition]
In technical terms, a blastocyst an embryo in a stage of development when the outer layer of cells has developed into a spherical structure and the inner cells have formed a dense clump with a fluid-filled cavity surrounding.
In simple terms, it looks like a tiny hollow ball with a small clump of cells and fluid inside.
Parts of a Blastocyst
- Outer cell layer (Trophoblast or Trophectoderm epithelium): These outer cells make up the outer shell-like layer of the blastocyst, and as they develop can become the placenta and supportive tissue for the fetus
- Inner Cell Mass (Embryoblast or Endoderm): This dense clump of cells within the spherical structure can eventually become the fetus
- Blastocyst Cavity (Blastocoele): This is a cavity inside the outer shell that is filled with fluid and holds amino acids and other molecules necessary for development
Zygote vs. Morula vs. Blastocyst Definition
The definition of blastocyst is best understood when you consider the stages of embryo development as a whole.
Zygote
A zygote is a fertilized egg. While it contains all of the necessary materials to become a fetus, it has not divided yet, and is only one cell.
Embryo
When a zygote divides, it becomes an embryo. It will continue to be called an embryo for several weeks of development.
Morula
As the embryo develops, it continues to divide and double the number of cells. It forms a clump of several cells called the morula. Healthy embryos in the morula stage tend to have cells with even shape and size.
Blastocyst
In the blastocyst stage, the cells in the embryo begin differentiating. That means that cells that once looked similar in size and shape will specialize. Some will become an outer shell-like layer that can eventually become the placenta, while some will form the Inner Cell Mass, a small clump within the spherical structure that can eventually become the fetus.
Benefits of Transferring Blastocysts
Embryos in the blastocyst stage are commonly preferred for transfer over earlier-stage embryos. The reasons for this can include:
- Better embryo selection based on embryo quality
- Higher implantation rates
- Less embryo transfers may be needed for success
- Can be frozen for future use
- Genetic testing is available
Blastocyst Grading and Transfer Success Rates
Embryos are commonly transferred while they are in the morula stage or the blastocyst stage, or on day 3 or day 5.
Early embryo grading (day 3) is relatively simple, with the number of cells and the shape and size of the cells being the main factors that determine grading.
For blastocysts, the grading can include:
- Quality of ICM (Inner Cell Mass): The inner cells become the fetus, and clinics check the appearance of this area.
- Quality of Trophectoderm: The cells that form the trophectoderm can develop into placenta and other tissues that are necessary for pregnancy. Good early development can mean higher likelihood of success.
- Blastocyst Expansion: As the embryo grows, a fluid-filled cavity forms (called a blastocoel). More developed or larger blastocoels are used as markers of good quality embryos.
Learn more about embryo grading and success rates.
Why Transfer Blastocysts vs. Day 3 Embryos?
One of the most common times for an embryo to stop developing is between the morula and the blastocyst stage (or between day 3 and the blastocyst stage). Once an embryo makes it to day 5 and the blastocyst stage, there is a higher likelihood the embryo will become a pregnancy and eventually a baby.
So blastocyst transfers tend to be more successful than early transfers.
That being said, it is possible that transferring an embryo on day 3 may enable the embryo to be more likely to develop into a blastocyst. A day 3 transfer may be ideal for those who do not have many embryos, because the nurturing uterine environment may be more suitable for these more fragile embryos to develop into a blastocyst, than a lab.
Talk to your physician to determine which transfer day is best in your situation.
For an even deeper dive into blastocysts, check out our guide.
If you’re creating embryos for surrogacy, our specialists can help you get started on building your family faster. When you have embryos created, we can help match you with a surrogate as soon as possible.
FAQ
What is the difference between an embryo and a blastocyst?
A blastocyst is an embryo in a specific stage of development. The blastocyst stage is a significant milestone in embryo development because this is often the stage when the embryo is transferred to the uterus and pregnancy can occur soon after.
Is a blastocyst considered a baby?
The blastocyst stage is an early stage of embryo development, and in many cases a blastocyst will not become a pregnancy or result in the birth of a baby. For a blastocyst to become a baby, it must be transferred to a uterus, develop into a pregnancy, then development must progress until the embryo becomes a fetus, and when development has progressed far enough, a baby can be born.
What is the difference between a blastula and a blastocyst?
The blastula is a part of the structure of a blastocyst. When a blastocyst forms, it looks like a mostly-hollow sphere with a smaller clump of cells within it. The spherical outside shell-like layer of cells is called the blastula or blastoderm.
Can you get pregnant with blastocyst?
Yes. Blastocysts are one of the most common forms of embryos to be transferred to a uterus in the hopes of achieving pregnancy. And when the transfer is successful and the blastocyst continues developing, a pregnancy often occurs.