PGD就是三代试管，全称是移植前基因诊断
PGD的类型有好几种，有单种基因检查，多种基因的检查。如果夫妇中一个携带某种遗传基因，用PGD可以筛选携带这种基因胚胎。早期流产60%是由于胚胎不正常，这些不正常胚胎的染色体大多是非整数陪体（aneuplody）。通常夫妇俩都没有染色体问题，但是卵子发育不好或精子有问题，配成的胚胎就会出现非整数倍体。PGD可以检查流产中胚胎常见的染色体非整数陪体，如三色体（trisomy ）21， 18， 13， 12， 9， 22。
胚胎如果是trisomy 21，18，13，12，9，22不正常，生命力会比较强，有些可以活到足月，有些不到三个月就停止。但是如果是别的trisomy，那就很短命了，比如trisomy 14，15，根本活不了几天。所以做囊胚，可以一定程度的筛除掉这些有trisomy问题的胚胎。
这是从一个生殖中心拷过来有关PGD的介绍
Preimplantation Genetic Diagnosis (PGD)
Developed in the early 1990's, preimplantation genetic diagnosis (PGD) is a way for couples to prevent a pregnancy affected by a genetic condition or chromosomal disorder. There are various types of PGD available, depending on the needs of the individual couple. This form of genetic testing is performed on eggs or embryos during an in vitro fertilization (IVF) cycle. The eggs or embryos that have been analyzed and are found to be normal are transferred into the woman's uterus, where, hopefully, they will implant and result in the birth of a healthy child.
Currently, we are able to perform PGD for many genetic conditions including single gene disorders and chromosomal abnormalities. At the Reproductive Genetics Institute, we have been performing PGD since it became available in 1990. We pioneered the polar body removal technology and are one of the most active centers offering PGD in the world. Our laboratory staff has extensive experience in the techniques involved with PGD.
How can PGD help me?
PGD can significantly reduce the chances for your baby to be affected with a specific genetic condition or chromosomal abnormality. We are able to test for many different diseases, including aneuploidy, single gene disorders and chromosome translocations.
Many couples request PGD for aneuploidy such as Down syndrome, trisomy 18, trisomy 13 and Turner syndrome. These disorders do not typically run in families. However, up to 60% of early miscarriages are due to aneuploidy, and the risk for aneuploidy increases with a woman's age. The purpose of PGD for aneuploidy is to increase a couple's chance for pregnancy, reduce their risk for miscarriage, and improve their overall chance of bringing home a healthy baby after in vitro fertilization.
Other couples request PGD for a specific genetic condition that may run in their family, such as Tay Sachs disease, cystic fibrosis, muscular dystrophy, Fragile X syndrome or spinal muscular atrophy. Our center has extensive experience testing for many single gene disorders, including rare genetic syndromes.
For individuals who carry a chromosomal translocation, PGD can be used to test eggs or embryos for their specific translocation. This greatly decreases the risk for miscarriage and/or the birth of a child with birth defects and mental retardation associated with an unbalanced chromosome translocation.
We can also test for the HLA status of embryos. This is requested by couples interested in having a child who is a healthy bone marrow match to another family member. HLA testing can be performed while testing for a specific genetic condition, such as beta-thalassemia.
What conditions are tested with PGD?
The conditions tested include:
Aneuploidy
Single Gene Disorders
Translocations
How is PGD performed?
Our geneticists use a variety of techniques for PGD including:
Polar Body analysis
Embryo analysis
At present, RGI is the only center in the world offering Preimplantation Diagnosis by sampling the first and second polar bodies. The first polar body is naturally discarded during oocyte maturation, prior to fertilization and contains one set of duplicated chromosomes (with two chromatids in each chromosome), while the second polar body is naturally discarded after fertilization and contains one set of chromatids. By testing the first and second polar bodies for a specific genetic disease, the geneticists can determine whether the woman's eggs carry the normal or abnormal copy of the corresponding gene. The procedure is offered to couples who are known carriers of a genetic disease detectable by DNA analysis. RGI physicians have already performed thousands of clinical cycles involving preimplantation diagnosis for cystic fibrosis, thalassemia, Tay-Sachs disease, sickle cell disease, and other single gene disorders, (click here for a list of all single gene disorders for which RGI has done PGD), resulting in many ongoing pregnancies confirmed to be unaffected by tests during pregnancy (CVS or amniocentesis) or after birth.
The other important application of the polar body testing is preimplantation diagnosis of Down syndrome and other common aneuploidies. Many patients referred to an In Vitro Fertilization program (IVF) are of advanced maternal age (35 years of age or older at the time of delivery), which places them at elevated risk for conceiving a child with an extra chromosome and thus delivering a child with Down syndrome (extra chromosome 21) or other common trisomies (trisomy 13 and 18). Aneuploidies may also result in spontaneous abortions (miscarriage) or nonimplantation, considerably decreasing the chances of the patient achieving a pregnancy. Therefore, by testing for the conditions beforehand, we can decrease the chances for spontaneous miscarriage and failed implantation and increase the chance of achieving pregnancy. Polar body analysis involves the removal of the first and second polar bodies for testing by FISH. Only embryos with a normal number of the chromosomes that are studied (for example, chromosomes 13, 16, 18, 21, and 22) are transferred. The patients may choose which embryos they wish to be transferred or frozen. The procedure has been offered to thousands of IVF couples and resulted in the birth of hundreds of healthy children confirmed to be unaffected by CVS or amniocentesis. This has also improved the pregnancy rate, compared to patients of advanced maternal age without polar body analysis.
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