n IVF in the U.S., fertilized eggs are usually cultured to days 5 to 7. The main reason for culturing to days 5 to 7 is:
1. Continue to observe the developmental potential of the embryo. Only the third day embryos with potential will continue to develop into 5-7 day blastocysts, while the embryologist will grade the blastocysts according to the morphology of the embryo.
2, Pre-implantation genetic testing PGT (third generation IVF), scientific research has proved that 50-70% of miscarriages are proved to be caused by chromosomal imbalance (too few or too many chromosomes) in the embryo, PGT test can solve chromosomal abnormality of the older children, blocking the family genetic diseases, effectively reducing the rate of miscarriage and substantially increasing the rate of live births;
It is important to note that sampling on day 3 embryos was banned in the US many years ago (affecting embryonic development), and it is safer to send samples from the blastocyst trophoblast (TE) on days 5-7.
3. Select blastocyst transfer according to the results for the highest live birth rate.
—
I. Blastocyst rating
The embryologist rates the blastocysts according to their developmental course, the morphology of the inner cell mass and the trophoblast cells (as shown in the table).
| Development | Stage 1: early chambered blastocyst, blastocyst cavity is less than 1/2 of total embryo volume; Stage 2: blastocyst cavity volume is greater than or equal to 1/2 of total embryo volume; Stage 3: fully dilated blastocyst, blastocyst cavity completely occupies total embryo volume; Stage 4: dilated blastocyst, blastocyst cavity completely fills with embryo, total embryo volume becomes larger, zona pellucida thins out; Stage 5: hatching blastocyst, some of blastocysts escapes from the zona pellucida ; Stage 6: hatching blastocyst, all of the blastocyst escapes from the zona pellucida. |
| Classification of the inner cell mass – the inner layer of cells that will continue to develop into the “fetus | Grade A: many cells, tightly packed; Grade B: few cells, loosely packed; Grade C: few cells. |
| Classification of trophoblast cells – the outer layer of cells that will continue to develop into the “placenta”. | Grade A: the epithelial cell layer consists of a large number of cells and has a dense structure; Grade B: the epithelial cell layer consists of a small number of cells and has a loose structure; Grade C: the epithelial cell layer consists of a sparse number of cells. |
Developmental Stages Stage 1 to Stage 5 Diagram
(picture from The magazine inviTRA)
We usually see 5AA, 3AA, 6AB in the report of blastocyst grading: the first number represents the stage of development of the blastocyst; the other two letters, the first one indicates the mass of the inner cell mass that will develop into the fetus, and the second letter indicates the mass of the trophoblast cells that will develop into the placenta, with the highest quality level of A, followed by B and C. Meanwhile, the number of days for the formation of the blastocysts is also an important reference The number of days of blastocyst formation is also an important indicator.
—
II. PGT testing
PGT tests are categorized as PGT-A, PGT-M, and PGT-SR. Normally, most people are tested only for PGT-A. The latter two tests are performed on blastocysts when there is a family history of a genetic disorder, or if the couple’s genetic screening is abnormal prior to IVF.
| 1. Preimplantation genetic testing for aneuploidy PGT-A | The most common chromosomal problem in preimplantation embryos. When an embryo has an incorrect number of chromosomes (gained or lost), it can lead to implantation failure, miscarriage, or chromosomal disorders such as Down syndrome (extra copies of chromosome 21). 50-70% of miscarriages have been shown to be caused by a chromosomal imbalance (too few or too many chromosomes) in the embryo. |
Corresponds to the original name PGS. The probability of aneuploid embryos increases with the age of the woman. 75% of women’s eggs are chromosomally normal at 25 years of age, about 50% at 35 years of age, and about 10-15% at 40 years of age. |
| 2. Pre-implantation genetic testing for single gene/single gene defects PGT-M | GT-M was first developed in the early 1990s to screen embryos for X-linked disorders such as fragile X chromosomes. The technique has been refined to include any inherited genetic disorder with a known mutation, such as cystic fibrosis, Tay-Sachs disease, muscular dystrophy dystrophy, and, more recently, hereditary cancers. Depending on the parents, they have a 25 %或50% chance of being passed on to the embryo. |
Detection of structurally abnormal parts of embryonic chromosomes. Corresponds to the original name PGD |
| 3.Pre-implantation genetic testing for chromosomal structural rearrangements PGT-SR | For couples in which one of the partners has a chromosomal rearrangement, such as a translocation or inversion, the test looks for any unbalanced form of these rearrangements, thereby reducing the chance of miscarriage and the chance of having a child with a chromosomal abnormality. |
With the current state-of-the-art NGS technology (PGT-A testing technology), it is possible to detect how many autosomes and sex chromosomes (X chromosome, Y chromosome) are present in each of the chromosomes from number 1 to 22.
he results of the NGS analysis are shown below. Usually, all autosomes are 2, and the cases with normal autosomes are shown in the baseline position. Abnormal chromosomes, 3 (Trisomy) are shown in the blue line; chromosome 1 (Monosomy) is shown in the red line; Mosaicism, which is the phenomenon of mixing chromosomally normal cells and chromosomally abnormal cells in a blastocyst, is shown in the middle of the two lines.
The 23rd pair of sex chromosomes, two X chromosomes, XX girls are shown below the red line; one each of X and Y, XY boys are shown at the red line.
Figure 1.
Chromosomally normal Boys
Chromosomal abnormalities Sex chromosome abnormalities
Trisomy of chromosome 9, monosomy of chromosome 18, sex chromosome abnormalities (2 X chromosomes, 1 Y chromosome, total of 3)
Chromosomal abnormalities Girls
Mosaicism #3 Trisomic Mosaicism (located between the baseline and trisomic lines) 16 Monosaicism (located between the baseline and monosomic lines)
—
III. Live birth rate
Since the beginning of IVF technology, people have been studying what kind of embryos have more developmental potential, what kind of embryos are healthier, and selecting better embryos for implantation to increase the live birth rate. And blastocyst rating and PGT-A testing are two methods commonly used in the United States today. With the PGT-A report, the
1) Blastocysts that do not pass PGT-A are usually just discarded by the clinic, regardless of the grade of the blastocyst.
2) Blastocysts that pass PGT-A will be evaluated for live birth rate based on blastocyst grade and selectively transferred. For example
Live birth rate 70% and above: 5AA, 4AA on day 5, followed by 5BA, 5AB, 5BB, 4BA 3AA 3BA
Live birth rate 65% and above: 4AB 4BB 3AB on day 5 followed by 3AB 3BB 6AA 6BA 6AB 6BB
There are also PGT-A blastocysts with live birth rates of 60 %、55%, 50% and below ……..
For example, PGT-A report 1: 5 blastocysts, 2 normal (Euploid, 1 male, 1 female). The live birth rate for a girl with 4BB on day 5 is 65% and above, and for a boy with 4AA on day 6 is 60% and above, making him an option for transfer.
For example, PGT-A report 2: 5 blastocysts, 3 normal (Euploid, 2 male, 1 female). The order of high live birth rate is: 4AA 3AA 4B+B.
The discarded 4BA, although the blastocyst grade is in the first tier, because of the trisomy 16 abnormality, the embryos may suffer from severe birth defects, growth retardation, and difficulty in existing past the first trimester, etc., please refer to the U.S. National Library of Medicine information https://www.ncbi.nlm.nih.gov/pmc/articles/ PMC9082733/), and no one is willing to try it on their stomach.
In general, the higher the grade of the blastocyst, the higher the probability of passing the PGT-A test, but it does not mean that all blastocysts with a high grade will pass the PGT-A test; at the same time, blastocysts with a normal PGT-A test can be attempted for implantation, regardless of their grade. It is also important to note that there are no official industry conclusions regarding the management of Mosaicism blastocysts. Depending on the degree, the possibility of a healthy baby being born exists.
我是分割线
