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Posts Tagged ‘chromosomes’

A new non-invasive test for detecting Down syndrome was announced recently and reported in this NY Times article.  This new test by Sequenom tests for Down syndrome by testing a sample of the mother’s blood rather than the more invasive tests of amniocentesis or CVS (chorionic villus sampling) which are the routine tests currently performed to check for chromosomal abnormalities, both of which carry risk of miscarriage.

According to a recent study published by the journal Genetics in Medicine, this new test was 98.6% accurate in picking up Down Syndrome cases, with a less than .02% risk of a false positive.  However, in the cases where Down Syndrome is detected, further invasive procedures that test all chromosomes are often recommended for additional confirmation.

Because the test does not pose a risk to the fetus and because it can be used as early as 10 weeks of pregnancy (earlier than both other current tests) experts believe it may result in fewer of the more invasive procedures being performed.  The drawback is that the test is not able to detect other chromosomal abnormalities, including a few rare forms of Down Syndrome that do not have three copies of chromosome 21.

The controversy (and potential medical ethics question) that arises is whether tests such as these will lead to more pregnancy terminations when Down Syndrome is detected, which would lead to diminished support and services for those currently living with this condition.

Two other companies have announced plans to release similar tests in 2012, and with the technologies that are being utilized for genetic analysis in these tests, we can expect future tests that may make it possible to prenatally diagnose abnormalities that do not involve extra chromosomes.

Read more at:http://www.nytimes.com/2011/10/18/business/sequenom-test-for-down-syndrome-raises-hopes-and-questions.html

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Well now, that’s a mouthful.  It’s no wonder the medical community comes up with all of these acronyms.  PDG – so much easier to say.   This is also sometimes referred to as PGS – pre-implantation genetic screening.  So, we continue our discussions of genetic testing. My last blog entry was about pre-conception testing, this one is focused on the most common form of testing done after an embryo is growing, but before it is transferred back to the uterus (usually on day 5 or 6). 

So, what is PGD and when would someone decide to use it? 

PGD can only be used when the embryos are fertilized and grown outside of the uterus – like in IVF.  The process of PGD is that the embryologist will take a biopsy of a growing embryo (usually on day 3) and remove one or two cells then usually they send it off to a lab that can test select chromosomes of those cells for a variety of genetic disorders and have the results back before the patients decide which embryos to transfer. This way you are eliminating the embryos that you know have genetic defects(not the standard 23 sets of chromosomes) and are only transferring embryos that are considered “competent” or without defects for the chromosomes tested. 

Who would want to do this testing?

There are a few main reasons that a patient may elect to have PGD testing:

  1. It can improve the likelihood of a successful pregnancy for patients who are at risk for passing along an inherited genetic disease.  This can be for something passed from either the egg or sperm.
  2. It can also be helpful for couples experiencing recurring miscarriages – it can rule out embryo competency as a factor for failure, or alert couples if a higher than expected number of embryos have chromosomal defects to allow for futher testing. 
  3. It can be used to help older women select the best embryos.  In patients with advanced maternal age (starting as early as our late 20s, but really making an impact after our mid-30s), we know the possibilities for chromosomal abnormalities in a woman’s eggs increases as we get older.  And the older you are, the higher the risk for genetic/chromosomal defects.   This is evidenced in the success rates of IVF in young women vs. older women.
  4. Another reason some patients may choose PGD is for family balancing – aka gender selection.   Note: although this is legal in the United States, many other countries do not allow PGD for gender selection. 

PGD cannot guarantee success.  And because it generally tests only about 5 sets of chromosomes that provide the greatest risk for defects, there still are risks for other genetic defects that may be incompatible with life, or result in a baby with health issues.  Often, even if PGD testing is done, the pregnant patient may be counseled to considered CVS or amnio as part of routine pregnancy screening. 

So, while older patients (who are at higher risk that  their eggs that could result in chromosomal abnormalities) may see somewhat higher success rates by using PGD, when using young donor eggs, the success rates may actually drop slightly.  Ask your clinic if they keep statistics on patients similar to you who use PGD vs. those who do not – this may help with your decision making.

Sounds surprising, right?  The reality is that there can be a small number of embryos that do not survive the biopsy and will be lost in the process.   Additionally, there can be cases of misdiagnosis (where it is unclear whether the embryo is competent or may appear to be abnormal).  Some reserachers even suggest that a day 3 embryo may show chromosomal issues that perhaps would be able to self-correct – but there is very little data to date to support this theory.  Additionally, there is some evidence that would suggest a PGD embryo may have a slightly lower likelihood of implantation compared to an embryo that has not been biopsied. 

Often cost can be a significant consideration in deciding whether to do PGD testing.  In the US, PGD testing can cost around $4000 -$5000 or more (depending on which set of chromosomes are tested), in Europe the costs start around $2000 and increase depending on the number of embryos tested and what testing is done.  This can add a significant expense to an IVF cycle – so it is important to understand the risks and benefits for your specific case. 

For all of these reasons, PGD is a procedure that should be discussed with your RE (reproductive doctor) so that together you can come up with the best plan for your particular case and circumstances.

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