Chromosomal abnormalities affect one in 200 babies. Most of us have 23 pairs of chromosomes—23 single chromosomes from our mother that pair with 23 from our father. However, babies can be born with too many or too few chromosomes, which can result in birth defects.

A trisomy is when there are three copies—instead of two—of a specific chromosome. Down's syndrome , one of the most common chromosomal abnormalities, occurs when a baby is born with an extra chromosome 21. Trisomy 18, which is much less common, occurs when a baby is born with an extra chromosome 18. While it is common for people with Down's syndrome to live to age 50 and beyond, they suffer from significant intellectual and physical disabilities. Babies born with trisomy 18 generally die by age one.

Expectant mothers have the option of taking the “triple screen” blood test to help identify Down's syndrome and other birth defects. This test is generally performed during the second trimester—between 16 and 18 weeks into the pregnancy. If the triple screen result is abnormal, further testing is recommended. Early detection of a birth defect allows a couple to either prepare for the birth of their baby or choose to end the pregnancy.

But some health professionals argue that the second trimester is too late for prenatal screening tests. Earlier screening could provide parents with greater privacy, earlier results, and the option of more safely terminating the pregnancy. A study in the October 9, 2003 issue of the New England Journal of Medicine reported that first trimester screening detects almost 80% of Down's syndrome cases, with a false positive rate of only 5%.

About the Study

The researchers screened over 8,000 pregnant women between 74 and 97 days of gestation (approximately 10-14 weeks into the pregnancy). The screening test calculated the risk of Down's syndrome and trisomy 18 based on the mother’s age, chemical markers in the blood, and nuchal translucency by ultrasound (the degree of swelling at the back of the fetus’ neck). Results were considered abnormal if the risk was determined to be greater than 1 in 270 for Down's syndrome, or greater than 1 in 150 for trisomy 18.

The researchers tracked the outcomes of the pregnancies and calculated the percentage of Down's syndrome and trisomy 18 cases that were detected. They also determined the number of false positive screening results (indicating an increased risk of Down's syndrome or trisomy 18 when the risk was not actually increased).

The Findings

The researchers found that the screening test identified 85% of the 61 Down's syndrome cases that occurred, with a false positive rate of 9%. When the researchers adjusted the false positive rate to 5% (the “gold standard”), they found that 79% of Down's syndrome cases would still be detected. The screening identified 91% of fetuses with trisomy 18, with a 2% false positive rate.

How Does This Affect You?

These results suggest that the first trimester screening test detects even more abnormalities than the second trimester triple screen. The triple screen detection rate for Down's syndrome is approximately 65%, compared to 79% with this first trimester screening (both based on a 5% false positive rate).

Many women who become pregnant after the age of 35 elect to screen for Down's syndrome and other trisomies because older mothers are at increased risk for these abnormalities.

A screening test like this one can help expectant mothers and their physicians decide whether or not more invasive testing, namely amniocentesis , is necessary. If prenatal screening does not identify an abnormality, some mothers may choose to avoid invasive testing, since it poses a small risk of miscarriage . Earlier screening can provide mothers with more time to make these decisions.

It is important to note that an abnormal screening result does not necessarily mean that there is a problem with the fetus. This is especially true of the triple screen test (not involved in this research). In fact, according to the March of Dimes, out of every 1,000 women who take a triple screen test, up to 100 will have an abnormal result, but only two or three will have a fetus with a birth defect. An abnormal result usually just means that the fetus is a few weeks older or younger than originally thought. It can also indicate a multiple pregnancy (twins, triplets, etc.).