A blastocyst is a fertilized egg, or zygote, on day 5–6 after fertilization, when it typically has around 100 cells. A blastocyst can be characterized as a rapidly dividing ball of cells. A blastocyst is rated on a scale of one to six depending on its size, with a rating of 1 or 2 indicating it contains fewer cells. A blastocyst “graduates” into an embryo after it implants in the uterus.

What is a blastocyst?

A blastocyst is a fertilized egg with around 200 cells (typically 5–7 days post-fertilization). At this stage of development, a blastocyst contains several distinct structures:

• the inner cell mass, which will eventually become the fetus and the amniotic sac.
• the trophectoderm, a layer of cells that will later develop into the placenta.
• a central cavity (the blastocoel cavity) that is filled with liquid.

When it’s first developed, the blastocyst will still have its zona pellucida, an outer shell that prevents additional sperm from penetrating the fertilized egg. The blastocyst will begin “hatching” from its outer shell 5–9 days after fertilization; removing the outer shell allows a blastocyst to attempt to implant in the lining of the uterus.

During implantation, the blastocyst starts to secrete the hormone hCG, which can be detected in the birthing parent’s blood and urine. The presence of hCG is what makes a pregnancy test positive. When a blastocyst develops an amniotic sac, around day 10–12 of gestation, it becomes an embryo.

How a blastocyst is created

When a sperm fertilizes an oocyte (egg), their nuclei combine to create a single cell called a zygote. Over the next few days, this zygote will undergo a series of cell divisions, progressing through 2-cell, 4-cell, 8-cell, and 16-cell stages.

During this progression, the cells are known as blastomeres. After the development of the 16-cell embryo has formed, the blastomeres begin to join to create inner and outer layers (the inner cell mass, trophectoderm, and blastocoel cavity mentioned above). Once fluid has accumulated inside the structure, it enters the blastocyst stage.

Male fertility and the blastocyst

The development of a healthy blastocyst is critical to pregnancy, whether through natural conception or fertility treatment. Research has shown that the quality of sperm is a good predictor of blastocyst formation, and in cases of in vitro fertilization (IVF), sperm quality had a significant impact on the likelihood of producing at least one blastocyst during the cycle.Another study that analyzed the relationship between sperm quality and blastocyst formation in intracytoplasmic sperm injection (ISCI) found that poor sperm health had a negative relationship with embryo development, including the early stages of blastocyst formation.