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Screening for Chromosomal Abnormalities

Common chromosomal abnormalities

The three most common chromosomal abnormalities which are screened for in the first trimester of pregnancy are:

Down Syndrome

The commonest chromosomal abnormality in which there is 3 copies of chromosome number 21 in all cells rather than the normal 2 copies.

Edwards Syndrome

When there are 3 copies of chromosome number 18 in all cells rather than the normal 2 copies.

Patau Syndrome

When there are 3 copies of chromosome number 13 in all the cells, instead of the normal 2 copies.

Babies with Edwards Syndrome and Patau Syndrome will not survive long after birth due to the multiple abnormalities associated with this condition, and most will miscarry early in the pregnancy or will be still born. Babies with Down Syndrome however, will often go on to become children and nowadays adults living to middle age.

Any woman who has a baby has a risk of her baby having one of the above chromosomal abnormalities. The risk of this increases with age, as the quality of the egg from the ovary reduces as the mother gets older, increasing the chance that the egg will not divide properly before conception leading to an abnormal chromosome count.

Screening Tests

There are 2 screening tests available for assessing risk for a mother having a baby with Down, Edwards or Patau Syndrome. Screening tests will give a high risk or low risk result for the baby having an abnormal chromosome result.

These screening tests do NOT give an absolute 100% guarantee that the baby does or does not have one of the three chromosome problems mentioned.

The only way to obtain a 100% guaranteed answer with regards to a chromosome abnormality is to have either an amniocentesis, which involves placing a needle in the uterus to obtain a small sample of fluid from around the baby (amniocentesis), to culture the baby’s chromosomes to get a definitive answer. This test can only be performed after 15 weeks and has a miscarriage risk of about 1 in 200.

An earlier invasive test can be performed from 10 weeks gestation called a Chorionic Villus Sample, which involves placing a needle into the developing placenta to obtain some tissue to culture out the chromosome make up the pregnancy. This test is 98-99% accurate, it is not quite 100% accurate as the cells are taken from the placenta and not from the fluid surrounding the baby. While this test gives a much earlier result, it has a miscarriage risk of in 1 in 100.

As both of these diagnostic tests have miscarriage risks, they are usually reserved for people who are at high risk for a baby with a chromosome abnormality, be it because of advanced maternal age, or because one of the screening tests has returned a HIGH risk result.

The first trimester screen

This involves a blood test at between 9-12 weeks to check 2 hormone levels related to the pregnancy, PAPP-A and Free B hCG. This result is put together with an ultrasound measurement of the back of the baby’s neck, which is measured at around 12-13 weeks gestation. A risk factor is calculated from the blood test levels, the thickness of the nuchal fold measurement and the mother’s age.

A risk result of less than 1 in 300 is considered to be LOW RISK and reassuring it is UNLIKELY the baby will have a chromosome abnormality.

The non-invasive perinatal test

This test is a newer test that has grown in popularity over the last 3-4 years. A blood test is taken from the mother’s arm at 11 weeks, and DNA fragments being released from the pregnancy into the mother’s blood stream and extracted and cultured out to check for chromosome abnormalities for Chromosome 21 (Down’s Syndrome), Chromosome 18 (Edwards Syndrome), Chromosome 13 (Patau Syndrome), and the two sex chromosomes (yes, you can find out the sex of your baby at 11 weeks gestation!).

The accuracy of this test is quite good. There is a 99.7% accuracy that a low risk test result will correctly rule out a baby with Down Syndrome. The accuracy for ruling out a baby with Edwards or Patau Syndrome is a little lower, at 97%, but still quite good. As babies with Edwards and Patau Syndromes have multiple anatomical abnormalities, having a 12 week scan to check for anatomical abnormalities also will increase accuracy of detection.

Remember that this test is not 100% accurate as it is not a sample directly from the uterus. A high risk NIPT result would need to be followed by an amniocentesis or CVS.