When is a blastocyst called an embryo

The fertilized egg zygote divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells.

Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes. About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick.

The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously. The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother.

Some of the cells from the placenta develop into an outer layer of membranes chorion around the developing blastocyst. Other cells develop into an inner layer of membranes amnion , which form the amniotic sac.

When the sac is formed by about day 10 to 12 , the blastocyst is considered an embryo. The amniotic sac fills with a clear liquid amniotic fluid and expands to envelop the developing embryo, which floats within it. The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side. This stage is characterized by the formation of most internal organs and external body structures.

Most organs begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization. At this time, the embryo elongates, first suggesting a human shape. Shortly thereafter, the area that will become the brain and spinal cord neural tube begins to develop.

The heart and major blood vessels begin to develop earlier—by about day The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day.

Blood vessels continue to develop in the embryo and placenta. Almost all organs are completely formed by about 10 weeks after fertilization which equals 12 weeks of pregnancy. The exceptions are the brain and spinal cord, which continue to form and develop throughout pregnancy.

Most malformations birth defects occur during the period when organs are forming. During this period, the embryo is most vulnerable to the effects of drugs, radiation, and viruses. At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks. The placenta forms tiny hairlike projections villi that extend into the wall of the uterus. Blood vessels from the embryo, which pass through the umbilical cord to the placenta, develop in the villi.

A thin membrane separates the embryo's blood in the villi from the mother's blood that flows through the space surrounding the villi intervillous space.

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Additionally, embryos that continue to grow to the blastocyst stage have a lower rate of chromosomal errors and therefore a higher chance of resulting in a healthy baby. Blastocysts sizes are measured on a scale of 1 to 6. Early blastocysts, with fewer cells, are graded as 1 or 2. Because these have fewer cells and a poorly defined inner cell mass and trophectoderm see below , they are not given any letter grades like the more advanced blastocysts are.

Blastocysts that are further along in development are given expansion grades of 3 through 6, where 6 indicates an embryo that has completely hatched out of the zona pellucida shell. This group of cells is given a letter grade of either A, B, or C. Grades A and B are most desirable and reflect a good number of cells with normal appearances.

Therefore only those cells graded A or B are selected to be cryopreserved. The trophectoderm represents the collection of cells that will become the placenta and are responsible for the initial invasion and implantation of the embryo into the uterine lining. This is also given letter grades of A, B, or C. Just as with the inner cell mass, grades A and B are most desirable. Though embryos are graded to signify their stage of development, it is important to remember that the grade does not always reflect the true potential of an embryo.