Treatments and investigations



The nucleus of almost every cell in the human body has a chromosome make-up (or karyotype) of 23 pairs of chromosomes making a total of 46. Half of each pair of chromosomes are inherited from the mother and half from the father. (The exceptions are mature red blood cells which do not contain a nucleus and eggs and sperm which only contain half the complement of chromosomes). The first 22 pairs are of chromosomes are called autosomes while the 23rd pair are the sex chromosomes (X or Y). Every egg contains an X chromosome, while a sperm may carry either an X or Y chromosome. A female will have two X chromosomes and a male an X and a Y chromosome. The sex of an individual will therefore depend on whether an X bearing or Y bearing sperm fertilises the egg. A female has a 46XX karyotype and the male 46XY.

Sometimes there can be an abnormal number of chromosomes in a cell. There may either be additional or missing chromosomes. This is known as aneuploidy and accounts for the majority of inherited disease syndromes.

When there is an extra chromosome instead of a pair this is called trisomy. In a live birth, he commonest trisomy to occur among the 22 pairs of autosomes is trisomy 21 (Down Syndrome). Trisomy can affect the sex chromosomes too: 47XYY (Klinefelter's Syndrome).

Monosomy is the term used when there is a missing half to a pair of chromosomes. An example of this is Turner’s Syndrome where there is a missing sex chromosome (45X0).

Rarely there can even be four or five copies of a chromosome.

When there is aneuploidy, this does not necessarily affect every cell. This patchy distribution of unaffected and affected cells is called mosaicism. To complicate matters further, there can be partial aneuplody where there is a genetic imbalance caused by the addition or loss of only a part of a chromosome, a situation referred to as unbalanced translocation.

It is possible to detect chromosomal abnormaities by analysing the pattern of chromosomes in cells. This is called karyotyping. There are a number of other refined techniques used in the genetic screening for aneuploidy, in particular Fluorescence Iin Situ Hybridisation (FISH), Comparative Genomic Hybridisation (CGH) and Quantitative Polymerase Chain Reaction (PCR).

In pregnancy it is possible to screen for aneuploidy by means of amniocentesis where a sample of the amniotic fluid around the baby is removed for FISH and karyotyping, and chorionic villus sampling where a small sample of tissue from the placenta is removed for testing. However there are a number of problems linked to such prenatal screening. These techniques are not without risk. Miscarriage can complicate these invasive tests and statistically when this shattering event happens it is far more likely to be a normal pregnancy that is lost than an affected one.

What is PGS

Embryos affected by aneuploidy frequently either fail to implant or miscarry. To go through miscarriage after miscarriage, or to have to come to terms with having a longed for pregnancy terminated because a significant abnormality has been found at pre-natal screening are soul-destroying experiences.

While pre-natal screening can detect those pregnancies affected by aneuploidy, PGS offers a screening technique that avoids the transfer of embryos that have common chromosomal abnormalities.

Possible indications for PGS

There are situations when there is a greater risk of aneuploidy occuring:

  • Women over the age of 35 years;
  • Women who have had recurrent miscarriages (3 or more) following embryo transfer;
  • Women who have had several failed full attempts at IVF (which include embryo transfer);
  • Women who have a family history of aneuploidy;
  • Male partners whose sperm show higher than normal levels of aneuploidy.
How is PGS performed?

IVF is carried out in the normal way (see IVF information). By day 3 the embryo will contain 6-8 cells (blastomeres). The embryologist removes one blastomere for chromosome analysis using the FISH screening technique. Clinics will be licensed to screen 5-10 specific chromosomes of the 24 chromosomes, looking for the commonest chromosome disorders. Unlike PGD (Pre-Implanation Genetic Diagnosis), PGS is not looking for a specific disorder (see Pre-Implanation Genetic Diagnosis information).

Embryos that are free from chromosome abnormality can then be transferred. A mavimum of two screened embryos can be transferred regardless of the age of the woman. If there are any suitable additional screened embryos these can then be stored for future use by freezing (see Cryopreservation of embryos information).

Some clinics will be licensed to remove cells from blastocysts (5-6 day old embryos). In the blastocyst cells are begiininjg to divide into those that will form the placenta (trophectoderm) and those that will develop into the fetus (inner cell mass). As blastocysts contain 100-150 cells as opposed to the 6-8 cells of a day 3 embryo, more trophectoderm cells (usually about 5) can be removed without having any detrimental effect.

The main disadvantage of using blastocysts for PGS is that not all embryos will reach the blastocyst stage resulting in a smaller group of embryos that are available for screening purposes.

It is also possible to carry out aneuploidy screening on unfertilised eggs by biopsying a small cell called a polar body that the mature egg pushes out of the egg cell. The advantage of testing the polar body is that this is not required for fertilisation and normal embryo development. In some countries (eg Germany) this gets round the ban that exists to screen pre-implantation embryos. The disadvantage of polar body screening is that such testing only gives information about the egg and with only one cell to test errors in diagnosis can occur. Work has been done to biopsy what is known as the second polar body which only develops after fertilisation has occurred.

Blastomere and polar body biopsy requires a high level of technical skill and experience. These techniques ay only be carried out by embryologists who have been appropriately approved by the HFEA.

Clinics are only permitted to test for those chromosomes for which the HFEA has issued a licence. Some clinics have been licensed by the HFEA to carry out comparative genomic hybridisation (CGH) which enables aneuploidy screening to be carried out on all 24 chromosomes.

It is not permitted to use PGS for sex selection purposes to try and achieve a balanced family.

Success rates of PGS

The figures below include PGS results for the whole range of indications. The figures for women over the age of 35 are likely to be different from the results for recurrent miscarriage.

The average success rate for PGS treatment in the UK for 2004 - 2006 is:

  • 33.9% (20/59) for women aged under 35
  • 30.9% (30/97) for women aged between 35-37
  • 25.5% (25/98) for women aged between 38-39
  • 17.3% (43/249) for women aged between 40-42
  • 8.8% (8/91) for women aged between 43-44
  • ** (0/21) for women aged over 44

** Percentages are not calculated where there are less than 50 cycles. Figures given in brackets are (cycles resulting in a live birth / all cycles started)

Figures by courtesy of the HFEA

So what are the problems with PGS?

The original thinking behind PGS was that all the cells or blastomeres of an embryo had the same chromosome pattern. Therefore if a single cell was screened it would show whether or not all the remaining cells in that embryo had a chromosome abnormality.

The discovery of mosaic embryos means that the cells of an embryo may not have identical chromosome patterns. Some cells may be normal and oher cells in the same embryo show aneuploidy. Indeed it can be shown that the majority of embryos will exhibit mosaic patterns and yet may develop into entirely normal pregnancies.

This means that many mosaic embryos will be rejected for transfer even though the detected mosaicism may be of no biological significance. The more refined screening techniques like CGH will detect more chromosome abnormalities than FISH. This will further reduce the number of embryos available for transfer.

It must be noted that the HFEA does not permit the transfer of screened and unscreened embryos together.

It is important to appreciate that that there is no robust evidence to demonstrate that PGS can be used to improve success rates in the categories of patients / indications that the technique is currently licensed for. While there are a number of studies that support the use of PGS, there are others that conclude that PGS makes no difference to live birth outcome. More robust randomised controlled trials are needed to assess whether PGS can significantly increase live birth rates for all indications.

One valuable aspect of PGS is that it gives patients an understanding of what the problem may be behind the failure of implantation or recurrent miscarriages. It may help patients to come to terms with the situation and even allow them to consider the alternative option of using donor eggs.

Before embarking upon PGS it is possible to obtain genetic counselling through the assisted conception unit. The clinical genetics specialist is likely to recommend that pre-natal screening should also carried out if a pregnancy occurs. It is appreciated that this can only add to the stress and worry that will already be experienced during pregnancy.

March 2009