What is prenatal screening?

Prenatal screening is a simple blood test done in pregnancy of 10 weeks or more to identify babies with serious fetal defects such as common chromosome abnormalities called trisomies (Down syndrome, trisomy 18 and trisomy 13) and open neural tube defects (spinal bifida and anencephaly).


Humans have 23 pairs of chromosomes, which are strands of DNA and proteins that carry genetic information. A trisomy is a chromosomal condition that occurs when there are three copies of particular chromosomes instead of the expected two.

Trisomy 21 is due to an extra chromosome 21 and is the most common trisomy at the time of birth. Trisomy 21, also called Down syndrome, is associated with mild to moderate intellectual diabilities and may also lead to digestive disease and congenital heart defects.

Trisomy 18 is due to an extra chromosome 18. Trisomy 18, also called Edwards syndrome, is associated with a high rate of miscarriages. Infants born with trisomy 18 often have congenital heart defect as well as various other medical conditions, shortening their lifespan.

Trisomy 13 is due to an extra chromosome 13. Trisomy 13, also called Patau syndrome is associated with a high rate of miscarriage. Infants born with trisomy usually have several congenital heart defects and other medical conditionss. Survival beyond the first year is rare.


Open neural tube defects (ONTD’s)

The most common neural tube defects are anencephaly and spina bifida.


Fetuses with anencephaly have a large part of the skull missing, and the brain is not properly formed. Those affected always die either before, or shortly after birth. Anencephaly is an open NTD, and like open spina bifida is associated with an elevated level of AFP in the maternal serum.

Spina bifida

Baby with spina bifida have an opening in the bones of the spine which can results in the damage to the nerves controlling the lower part of the body. This can cause weakness and paralysis of the legs and sometimes inability to control the bowel and bladder.

Prenatal screening detects trisomies of chromosomes 21, 18 and 13 and open nueral tube defects (ONTD’s) in the fetus, but does not rule out other fetal abnormalities.

The risk of having a baby with a chromosomal abnormality increases with maternal age. However, because younger women have more babies, 80% of all chromosomal abnormalities are found in mothers less than 35 years of age.

Down syndrome is the most common chromosome abnormality among live births and the most frequent form of intellectual disability caused by a demonstrable chromosomal aberration. The syndrome is characterized by moderate to severe learning disability (average IQ approximately 40) in combination with short stature, characteristic facial features, heart defects (40 to 50 percent of cases), intestinal malformations (10 percent of cases), problems with vision and hearing (50 percent of cases), an increased frequency of infection, and other health problems.