Fetal Chromosome Abnormalities
Normally, all cells in the human body have 46 chromosomes, made up of 23 pairs of chromosomes. 23 chromosomes (one of each pair) are inherited from the father and 23 inherited from the mother. These 23 pairs of chromosomes contain all of the DNA, or building blocks, to determine the health of the baby. If, at the time of conception, one extra chromosome is passed on, the baby will have a trisomy (one extra of a particular chromosome). If that extra chromosome is a number 21 chromosome, the baby will have Trisomy 21, more commonly called Down syndrome. If that extra chromosome is a number 18 chromosome, the baby will have Trisomy 18, more commonly called Edwards syndrome, while an extra chromosome 13 is Trisomy 13, more commonly called Patau syndrome. The risk of these abnormalities increases as the mother gets older.
Non Invasive Prenatal Testing (NIPT)
We now know that tiny amounts of the baby’s DNA is found floating in the maternal blood as early as 9 or 10 weeks gestation. A simple non-invasive blood test can measure this fetal DNA and provide an estimate of risk of common fetal chromosome problems. The purpose of the Non-Invasive Prenatal Test (NIPT) is to screen pregnancies to determine which ones are at high risk for the fetus to have extra or missing copies of the specific chromosomes 21, 18, 13, X or Y. The test is performed on a maternal blood sample which contains DNA (genetic material) from both the mother and fetus.
This screening test can detect up to 99% of the abnormalities of chromosomes 21, 18 and 13 and over 90% of cases of Monosomy X.
In the vast majority of cases (over 95%) the chances of passing on an extra chromosome are determined by the mother’s age. The older a woman is when she gets pregnant, the higher the chance that an extra chromosome will be passed on. It is important to realise that any woman can have a baby with Down syndrome, regardless of her age. For example, a 20 year old woman has about a 1 in 1,000 risk of passing on an extra chromosome causing Down syndrome, while a 40 year old woman has about a 1 in 70 chance of this happening. For most parents, the chances of having a baby with Down syndrome have nothing to do with family history or how healthy other pregnancies were.
Down syndrome is probably the most common cause of mental retardation. Babies with Down syndrome tend to develop more slowly than other babies do. For example, they may start walking later than other babies. About half are born with heart defects, or with a blockage in their intestines that prevents them from digesting food properly. These heart and intestine problems are usually fixed by surgery. Most children with Down syndrome go to regular schools, but special additional resources are usually made available as they grow and develop.
Like Down syndrome, this condition occurs when there is one extra chromosome, but in this case it is an extra chromosome number 18. Trisomy 18 is often also called Edwards syndrome. In the vast majority of cases, the chances of having a baby with Edwards syndrome are also related to the mother’s age. However, Edwards syndrome is much rarer than Down syndrome. For example, a 20 year old woman has close to a 1 in 2,000 risk of passing on an extra chromosome number 18, and a 40 year old woman has close to a 1 in 150 chance of this happening.
Another big difference between Edwards syndrome and Down syndrome is that Edwards syndrome is usually considered to be lethal, with very little possibility of the baby surviving. This is because most babies with Edwards syndrome have severe mental retardation, serious heart defects, and problems with development of the brain, spine, kidneys, or intestines. Most fetuses with Edwards syndrome are very small and don’t grow very well in the womb. About two-thirds of all fetuses with Edwards syndrome die in the womb before birth. A small number of babies born alive with Edwards syndrome may even survive for a few months, but this is quite rare.
Trisomy 13 is quite rare, occurring in about 1 in 5,000 births. Like Down syndrome and Edward syndrome, it is caused by one extra chromosome, this time a chromosome number 13. Also called Patau syndrome, the chances of it happening are determined by the mother’s age in most cases.
Like Edwards syndrome, Patau syndrome is usually considered to be lethal, with virtually no possibility of the baby surviving. Almost all babies with Patau syndrome have severe abnormalities, like serious heart defects, and problems with development of the brain, face, kidneys, limbs or intestines. Rare cases of Patau syndrome may survive for a few months after birth.
Problems Associated with Increased Nuchal Translucency Size
If the thickness of the back of the baby’s neck is quite big, this can be associated with fetal chromosomal abnormalities, but other fetal problems might also be present.
These problems are very rare, and may include fetal heart problems. This special early screening test also provides other very useful information:
- Accurate dating of the pregnancy
- Fetal heart rate
- Number of fetuses present and, if twins, whether they are identical or not
- Detects obvious fetal abnormalities
It is important to remember that a screening test does not guarantee a normal baby. Therefore you should carefully compare the advantages and disadvantages of a screening test (like NIPT or nuchal translucency measurement) with a diagnostic test (like CVS).
Only you can decide if you wish to have an invasive diagnostic test, like CVS, and you will always have the right to choose this definitive test if you prefer. It is also recommended that you have an anatomy scan at 20 weeks to check the baby so that we can reassure you about problems that the CVS cannot detect.