Prenatal Diagnosis of Down Syndrome
Prenatal diagnosis means trying to discover what medical problems a fetus has while it is still within the uterus. At the present time in the United States, the Triple, Quad or Penta Screen blood test is probably offered to most pregnant women to assess their risk of having a baby with Down syndrome or neural tube defects (spina bifida). This method of screening can lead to the diagnosis of about 65 % of the Down babies before they are born. The risk of having a Down baby increases with the mother’s age, but because most babies are born to young women, most Down babies are born to young mothers. Therefore all pregnant women, young and old, should be screened.
Most women in the United States have at least one ultrasound during their pregnancy, and of course ultrasound is another important method of prenatal diagnosis.
The diagnosis of Down syndrome and other chromosomal abnormalities is not made by the triple screen or quad screen blood tests, but rather must be made by amniocentesis or chorionic villus sample (CVS). The triple or quad screen blood test and ultrasound help us to decide who should have amniocentesis or CVS. At the present time in the United States the diagnosis of Down syndrome is most often made in the second trimester of pregnancy.
It is now possible to assess risk of Down syndrome and other chromosomal abnormalities (Trisomy 18, trisomy 13, and Turner’s syndrome) in the first trimester of pregnancy. This can be done by nuchal translucency done between 11 weeks 0 days and 13 weeks 6 days from the last menstrual period. Also a blood test done at the same time, plus factoring in the maternal age, can lead to the identification of about 90% of Down babies. This first trimester test is done .
Earlier identification of a baby with a chromosomal abnormality means a safer termination if that is elected, more time to prepare for the birth of a Down baby, more time to seek consultation.
The nuchal translucency thickness may be increased not only in babies with chromosomal abnormalities, but also in babies with major heart defects, and many other anomalies.
The first trimester risk assessment does not include neural tube defects, so it is still necessary to have an alpha fetal protein (AFP) test at about 16 weeks gestational age to assess risk of neural tube defects. A carefully done ultrasound at about 18-22 weeks may also rule out neural tube defects.
During the early ultrasound for nuchal translucency, at around 12 weeks, (11 weeks 0 days to 13 weeks 6 days to be exact) it is possible to date the pregnancy quite accurately, diagnose twins, and sometimes see major problems.
At the conclusion of this early assessment, the mother is given a risk of her having a baby with Down syndrome. This risk is given in the form of a ratio (1 chance in 250, for example, expressed as 1/250). We start with a risk based on the maternal age, and the gestational age (the risk of Down syndrome actually decreases as the pregnancy progresses, because many such pregnancies end in miscarriage). The results of the nuchal translucency measurement and the results of the blood tests then modify the age-related risk.