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A blastocyst is a ball of cells that forms early in a pregnancy, about five to six days after a sperm fertilizes an egg. It implants in your uterine wall, eventually becoming the embryo and then the fetus.
A blastocyst is a cluster of dividing cells made by a fertilized egg. It’s the early stage of an embryo. A blastocyst is one step among many that lead to a pregnancy.
A blastocyst forms about five to six days after a sperm fertilizes an egg. Layers of cells in the blastocyst divide and separate. They eventually become the structures that protect and nourish the developing fetus.
The blastocyst stage of a fertilized egg is especially important for in vitro fertilization (IVF). IVF is the process of creating an embryo outside of the birthing parent’s body to assist with pregnancy.
To understand the role of a blastocyst, it helps to know how ovulation, conception and pregnancy occur.
About 14 days after a person begins their menstrual period, their ovaries release one egg (ovum). The egg goes into one of their fallopian tubes, where it waits for fertilization by a sperm.
If fertilization doesn’t occur, the egg moves into their uterus where it exits their body during their next menstrual period. But if fertilization does occur, their fertilized egg becomes a zygote, a single cell with genes from both parents.
The zygote travels through their fallopian tube to their uterus about three to five days after fertilization. The cell of the zygote continuously divides, eventually forming a hollow ball of cells called the blastocyst.
The blastocyst stays in their uterus for several days before it implants in the inner lining of their uterine wall (endometrium). It continues to make new cells, which separate into layers. About 10 to 12 days after fertilization, the blastocyst develops into an embryo. It remains an embryo until about nine weeks after implantation, when it then becomes a fetus.
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The blastocyst stage is an extremely important part of embryonic and fetal development. If the blastocyst doesn’t implant in the person's endometrium, pregnancy will not occur.
For implantation to occur, hormones trigger a process called hatching. The blastocyst sheds its clear outer membrane. Hatching occurs one to three days after a blastocyst enters their uterus.
Next, cells on the outer layer of the blastocyst attach to the outer layer of their endometrium. They release a sticky protein called L-selectin, which binds with substances in their endometrium. These outer cells eventually become the fetus.
Cells in the inner layer of the blastocyst implant more deeply in their endometrium. These cells become the placenta, which carries oxygen and nutrients to the developing fetus. Some of the placental cells also form the amniotic sac. The amniotic sac contains a clear, protective fluid that surrounds the fetus as it develops.
Implantation usually happens toward the top of the person's uterus. In rare cases, a blastocyst never makes it to their uterus. Instead, it implants outside their uterus (usually in a fallopian tube), creating an ectopic pregnancy.
About one-third of people experience some light bleeding or spotting during implantation. Spotting may be the first sign of pregnancy. The blood tends to be light pink or dark brown and only lasts for a few hours to a few days, so it won’t be similar to your period.
Other signs may include:
During IVF, healthcare providers evaluate and grade blastocysts in a laboratory to determine which embryos are most likely to lead to a pregnancy. Five or six days after fertilization, blastocysts are ideal to transfer to your uterus during IVF. Healthcare providers may grade the blastocysts based on their maturity, shape, cell number and density.
A blastocyst is a hollow ball of cells. The cells form two layers. The inner layer is about three to four cells thick and the outer layer is about one cell thick.
The cells in a blastocyst divide rapidly. A mature blastocyst may contain as many as 200 to 300 cells.
Blastocysts contain chromosomes. If there are chromosomal abnormalities, such as an extra chromosome or a chromosome that’s duplicated or deleted, the blastocyst usually won’t implant in your endometrium. Failed implantation may lead to an early miscarriage at about five weeks, often called a “chemical pregnancy.” If you haven’t missed your period or taken a pregnancy test, you may not even know you had a chemical pregnancy.
If the blastocyst does implant and leads to a successful pregnancy, there is always a risk that the baby may have a chromosomal disorder such as:
Failure for a blastocyst to implant is one of the most common reasons for unsuccessful IVF treatments. It’s also the cause of about 75% of early miscarriages.
About half of all early miscarriages are the result of chromosomal abnormalities in embryos.
You may not know if a blastocyst fails to implant. A lot of people don’t have any symptoms. Some experience light spotting or bleeding. Since these symptoms are similar to those of successful implantation, it’s important to talk to your healthcare provider so they can test for pregnancy.
During IVF, healthcare providers can do preimplantation genetic testing (PGT). They take cells from an embryo at the blastocyst stage to check for chromosomal abnormalities. They perform tests in a lab to identify which embryos are healthy and have the greatest chance of implanting once they’re transferred to your uterus.
If you get pregnant naturally, prenatal testing for chromosomal abnormalities happens after the blastocyst stage. The earliest tests usually take place nine to 10 weeks into a pregnancy.
Many risk factors for chromosomal abnormalities aren’t preventable, such as:
If you’re at high risk due to any of these factors, talk with your healthcare provider and a genetic counselor if you plan to get pregnant.
You can lower your risk of causing chromosome damage in an embryo by practicing a healthy lifestyle before and during pregnancy:
A note from Cleveland Clinic
Blastocysts are part of early embryonic development and play a crucial role in a successful pregnancy. Blastocysts that don’t develop normally or don’t implant in the uterus won’t lead to a pregnancy. If you’re having IVF, your healthcare provider can test blastocysts to see if they’re healthy. You might experience light bleeding or mild cramping very early in a pregnancy as the blastocysts form and attach to your uterus. But call your healthcare provider right away if you notice more serious symptoms.
Last reviewed by a Cleveland Clinic medical professional on 04/29/2022.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy