What Is Down Syndrome

Using ultrasound to look for the nose bone in early pregnancy could help to identify Down babies sooner, new research suggests. Some experts say the test, described for the first time this week in The Lancet medical journal, could be a major step forward, bringing the detection rate of Down screening up from about 65% today to about 98% and allowing diagnosis in the first trimester rather than the second.

Because the method has a lower false positive rate than current screening tests, it could also reduce the number of women having to undergo unnecessary diagnostic tests, which involve inserting a needle into the womb and trigger miscarriages about 1% of the time, experts said.

"Adding the nasal bone test is a big leap forward," said Dr Howard Cuckle, professor of reproductive epidemiology at Leeds University in England who was not connected with the study. "But, the thing that's missing generally is this move to the first trimester."

However, other obstetricians said that while the findings were impressive, detecting nasal bone on an ultrasound would require special skills and it would be difficult to maintain quality measurements everywhere.

"In the US we are still not fully using first trimester screening, partly due to insurance reimbursement issues - most health insurers pay for only one routine sonogram," said Dr Joshua Copel, chief of maternal foetal medicine at Yale University School of Medicine.

Doctors are also waiting for the results of a major US study comparing first- and second-trimester screening techniques, he said.

One of every 700 children is born with Down, a chromosomal abnormality that is a common cause of mental retardation and is marked by a broad, flat face with slanting eyes.

The condition is caused by having three Chromosome 21, instead of two.

Prenatal diagnosis is made by one of two tests that analyse chromosomes in cells taken from the mother's belly with a needle.

The most popular test is amniocentesis, which draws out amniotic fluid during the second trimester of pregnancy.

The other, which is most widely used in Europe, is chorion villus biopsy, or CVS. The CVS test extracts a sample of the placenta and is performed during the first trimester.

Both tests trigger miscarriages about 1% of the time.

For many years, only women over 35 - who have a higher chance of a Down baby - were sent for a diagnostic probe, but there are now three additional screening methods to help determine who should get one.

The most common screening approach, which involves a blood test for certain red flags, is done in the second trimester of pregnancy. The standard practice in the United States, it detects about 65% of Down cases before they are confirmed by amniocentesis confirmed by amniocentesis.

Another technique, which relies on whether an ultrasound shows a shaded area suggesting a thickening at the back of the foetus' neck, is done in the first three months of pregnancy, between weeks 11 and 14. That test, used by some of the top centres in Europe, spots about 75% of cases.

The third approach is to combine both the foetal neck test and the blood test in the first trimester, which gives a detection rate just
under 90%.

In the study, led by Dr Kypros Nicolaides, head of foetal medicine at King's College Hospital Medical School in London, doctors examined 701 routine ultrasound scans done at 11-14 weeks for women about to undergo prenatal diagnosis in the first trimester because of positive results on a test of the thickness of the back of the foetus' neck.

The nasal bone was absent in 43 out of the 59, or 73%, of the foetuses that were later confirmed to have Down and in three of the 603 normal ones, or 0.5%. Adding the nasal bone screening to the neck thickness test pushed the detection rate up from 75% to 85%, while cutting the rate of false positives from 5% to 1%, the study showed.

Cuckle estimates that combining the nose bone test with the neck thickness scan and the blood test, all in the first trimester, could allow screening to detect almost 98% of Down cases.

It's a compelling argument in favour of shifting Down diagnosis policies from the second trimester to the first, he said.

"First-trimester screening has obvious benefits over second-trimester screening, other than efficiency" Cuckle said. "These advantages include, for some, an early diagnosis with consequent safer and less traumatic therapeutic abortion and, for most, an earlier reassurance." - AP

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