Understanding Fetal Abnormalities
Approximately 3% of all pregnancies would be affected by chromosomal, genetic or physical (structural) disorders. There are newer more accurate tests on the market at the moment, and most are able to detect abnormalities at an early stage of pregnancy.
1. FETAL CHROMOSOMAL ABNORMALITIES
Down Syndrome (Trisomy 21)
Down Syndrome is the commonest, and most widely known chromosomal abnormality affecting newborn babies. Persons with Down syndrome are born with three chromosome 21s instead of the normal two. That is why Down syndrome is also known as trisomy (tri = 3, -somy = chromosome body) 21.
Conditions vary and some Down Syndrome children have a higher degree of intellectual ability than others. Some children may have cardiac and gastrointestinal conditions as well.
The risk of having a Down Syndrome baby increases as a mother gets older. A new non invasive test is now available. The test is based on next generation sequencing technology called Massively Parallel Sequencing (“MPS”) that analyzes circulating cell-free fetal DNA extracted from a maternal blood sample, and measures how many copies of fetal trisomies are present. This is followed by an in-depth bioinfomatics analysis of fetal DNA ratios. It is able to detect with near 100% accuracy for Trisomies 21, 18 and 13.
Edward Syndrome (Trisomy 18)
Children with Edward syndrome have an extra chromosome eighteen. This condition is much less common than Down syndrome, but the spectrum of abnormalities is much greater including the face, the hands and feet, heart and other internal organs. Survival beyond the first year of life is unlikely.
Current non- invasive prenatal screening tests can detect Edwards Syndrome with 99% accuracy. If Edwards Syndrome is suspected, diagnosis can be confirmed by diagnostic tests such as amniocentesis or chorionic villus sampling.
Patau Syndrome (Trisomy 13)
Children with Patau syndrome have an extra chromosome thirteen. This condition is much less common than Down syndrome, but the spectrum of abnormalities is much greater.
Current non- invasive prenatal screening tests can detect Patau Syndrome with 99% accuracy. If Patau Syndrome is suspected, diagnosis can be confirmed by diagnostic tests such as amniocentesis or chorionic villus sampling.
Turner syndrome is a chromosomal disorder that affects girls. Females usually have two X chromosomes, but in Turner syndrome, one of the X-chromosomes is missing.
These children may have stunted growth and limited sexual development, but treatment is now available that would assist with sexual development and allow girls affected by Turner syndrome to achieve close-to-average heights.
2. FETAL GENETIC ABNORMALITIES
Thalassaemia is a genetic blood disorder that can be inherited from the parents. Usually the parents have a milder version of the condition, but if the fetus inherits two affected copies of the genes, then the newborn would have a severe form of the disease.
In all thalassaemias, not enough of a key type of globin is produced – either the alpha-globin or beta-globin chain. Both are essential for making haemoglobin, the main ingredient in blood.
There are two forms of thalassaemia that are common in Asia: alpha-thalassaemia and beta-thalassaemia. Each can lead to very different forms of disease. In alpha-thalassaemia, the entire gene goes missing, sometimes two genes on the same chromosome are completely absent. In alpha-thalassaemia, occasionally, the fetus is so badly affected that it may not make it to term. This lethal form of the disease is known as Bart Hydrops Fetalis, and occurs mainly in Asia.
Beta-thalassaemia happens because point mutations within the beta-globin gene affect the production of this essential protein. In beta-thalassaemia, some affected children need repeated blood transfusions.
In Singapore, all pregnancies are screened for the possibility of being affected by thalassaemia. If suspected early, special tests can be offered to the parents, and the fetuses can be investigated for the disease.
This is an inheritable genetic condition whereby the parents may not have any symptoms, but if the fetus inherits two copies of the affected genes, then the newborn would have a severe form of the disease. In this condition, the body produces exceptionally thick and viscous mucus, which affects the lungs and digestive system. The mucus can clog lungs and cause infection, and also prevent the pancreas from producing natural enzymes to digest food efficiently. Fetal testing requires prenatal diagnostic tests such as CVS and amniocentesis.
3. FETAL STRUCTURAL ABNORMALITIES
Neural tube defects
One of the commonest groups of structural (physical) abnormalities discovered in unborn babies is the neural tube defect. Of these, most parents would have heard of conditions such as hydrocephalus and spina bifida.
Hydrocephalus is a condition in which there is an excessive amount of fluid around the brain and spinal cord. Babies may either acquire this condition after birth, or be born with it. The prognosis depends on the underlying cause and severity of hydrocephalus.
Spina bifida is a condition where a baby’s vertebral column and spinal cord do not develop normally. Occasionally, the two sides of the vertebrae don’t fuse properly. The prognosis depends on the severity of the spina bifida.
Both these conditions can be diagnosed quite accurately by ultrasound examination during early pregnancy. They can also be prevented by the use of folic acid started before conception.
Congenital Heart Disease
Another common group of fetal structural abnormalities includes defects of the heart and large blood vessels near the heart. There are many recognized causes for alterations in the structure of the heart e.g. maternal diseases, use of certain medicinal products, and chromosomal and genetic diseases.
These days, congenital heart diseases are eminently correctable and many children grow up healthy and have relatively normal lives.