Blastocyst

A blastocyst is the early stage of an embryo. It’s a cluster of dividing cells that forms about five to six days after conception. The blastocyst is the stage an embryo must reach before it can implant in your uterus. It’s one step among many that lead to a pregnancy.

Cells in the blastocyst continue to rapidly divide and separate into two types. Some of the cells will develop into the fetus. The other cells become the placenta, which protects and nourishes the developing fetus.

The blastocyst stage of an embryo is especially important for in vitro fertilization (IVF). During IVF, the fertilized egg must grow in the culture dish to the blastocyst stage before it can go in your uterus. The IVF laboratory monitors embryo growth. Blastocyst quality is graded based on maturity, appearance and cell number. This helps determine which embryos are most likely to lead to a pregnancy.

How does it form?

There are a few steps before the blastocyst stage:

• A sperm fertilizes an egg.
• The fertilized egg becomes a zygote. It’s a single cell with genes from both parents. The zygote divides, creating an embryo with two cells.
• The two-cell embryo continuously divides, eventually reaching 80 to 100 cells. This ball of cells is the blastocyst.

The blastocyst stays in your uterus for several days before it implants in the inner lining of your uterus (endometrium). During IVF, blastocysts are ideal to transfer to your uterus five or six days after fertilization.

What is the purpose of the blastocyst?

The blastocyst stage is a very important part of embryonic and fetal development. If the blastocyst doesn’t implant in the lining of your uterus (endometrium), pregnancy won’t occur.

For implantation to happen, hormones trigger a process called hatching. The blastocyst sheds its clear outer membrane. Hatching occurs one to three days after a blastocyst enters your uterus.

The blastocyst consists of two types of cells: the ICM and trophectoderm. The inner cell mass (ICM) has a small number of cells. The outer layer of cells is the trophectoderm.

Cells on the outer layer of the blastocyst attach to your endometrium. They release a sticky protein that binds with substances in your endometrium.

These outer cells become the placenta. It 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.

The ICM cells of the blastocyst continue to rapidly grow and eventually develop into a fetus. This happens about six to seven weeks after implantation.

Where does blastocyst implantation occur?

Implantation usually happens toward the top of your uterus. In rare cases, a blastocyst never makes it to your uterus. Instead, it implants somewhere else (usually in a fallopian tube), creating an ectopic pregnancy. An ectopic pregnancy isn’t viable.

What are the signs of implantation?

About one-third of females experience some light bleeding 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. It won’t be similar to your period.

Other signs may include:

• Breast tenderness
• Headaches
• Lower back pain
• Mild cramps, which aren’t usually as painful as menstrual cramps
• Nausea

How is a blastocyst structured?

A blastocyst looks like a ball that has:

• A clear outer protective layer called the zona pellucida
• A ball of cells called trophoblasts that are under the zona pellucida
• An inner cluster of cells attached to one area of trophoblasts, known as the inner cell mass
• A cavity of fluid inside the blastocyst, known as the blastocele

The cells in a blastocyst divide rapidly. Prior to implantation, a blastocyst may contain 70 to 100 cells.

What causes blastocysts to fail to implant?

Each cell within the blastocyst contains chromosomes from both parents. If there are chromosomal issues, the blastocyst usually won’t implant in your endometrium. Failed implantation may lead to an early miscarriage at about five weeks.

Healthcare providers often call this 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’s still a chance that the fetus could have a chromosomal disorder, such as:

• Down syndrome
• Klinefelter syndrome
• Turner syndrome

During IVF, healthcare providers can do preimplantation genetic testing (PGT). They take cells from the blastocyst to check for chromosomal abnormalities.

What are the symptoms of a failed implantation?

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. These symptoms are similar to those of successful implantation. So, it’s important to talk to your healthcare provider so they can test for pregnancy.

How often do blastocysts fail to implant?

Failure of 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.