Prenatal biochemical screening, which is widely used to trace chromosomal abnormalities in the fetus, does not have a satisfactory success rate, Italian researchers have said. They added that such screenings can detect abnormalities just 50 percent of the time.
Chromosomal abnormalities in the foetus can lead to serious handicap, or even death during gestation or in the first few days after birth.
Dr. Francesca R. Grati of the TOMA Laboratory, Busto Arsizio, has revealed that the new finding is based on a study of 115,576 prenatal diagnoses carried out during the last fourteen years.
The researcher revealed that 84,847 were amniocenteses, usually carried out around the 16th week of pregnancy, and 30,729 chorionic villus samplings, which can be undertaken from 12 weeks into the pregnancy.
Since both tests carried an increased risk of miscarriage, the decision on whether or not to undertake them could be difficult to weigh up.
"Since our sample included a large number of women aged less than 35 who underwent invasive prenatal diagnosis without any pathological indication to do so, we felt that the results could be useful in helping to inform pre-test counselling of such women. Up until now, the information we had came from smaller studies which only looked at the performance of these tests in detecting a limited number of chromosomal abnormalities," said Dr. Grati.
The research team first analysed the results of the chromosomal abnormalities from their own dataset, and then combined them with the official detection rates for such abnormalities published by SURUSS and FASTER consortia - multi-centre research groups involved in the investigation of screening and diagnostic tests performed in pregnancy, whose results are being used to optimise prenatal care for pregnant patients.
The researchers observed that the existing screen procedures were only able to detect half the total chromosomal abnormalities in women both younger and older than 35.
Dr. Grati said that current tests could not detect all foetal chromosomal abnormalities, but only trisomies 21 (Down syndrome), 18 (Edward's syndrome), and 13 (Patau syndrome), monosomy X (Turner syndrome), and triploids (conceptuses with 69 chromosomes instead of 46).
"These are common vital chromosomal abnormalities, but there are many others which are not picked up by these tests. And the tests do not even detect 100 per cent of the common abnormalities," says Dr. Grati.
At conception, 23 chromosomes from each parent combine to create a foetus with 46 chromosomes in all its cells, and trisomy occurs when the foetus has one additional chromosome (47 instead 46). The extra genetic material from the additional chromosome causes a range of problems of varying severity.
Other than trisomy, the foetus may also have the loss of genetic material (deletions) or chromosomal abnormalities in a non-homogeneous form, where there is a mixture of two cell lines, one normal and the other abnormal.
"Some of these disorders are relatively common in the foetus, which may have as much chance of surviving as children who are born with Down syndrome, and it is worrying that current biochemical tests are not always able to detect them. Our research confirms that it is fundamental for doctors to counsel patients about the limitations of current screening methods, so that they can make an informed decision on whether or not to undergo invasive diagnostic testing," said Dr. Grati.