Recently there has been widespread publicity of a new method, called chorionic villi sampling, for detecting abnormalities in the unborn child. Cells are taken from the placental tissue (chorion) of the developing child and are studied for chromosomal abnormalities.
It has been thought that any chromosomal defects found in the placenta would also exist in the child. The procedure, now performed in many centres, can be done earlier than amniocenesis, thus permitting earlier abortion, however the accuracy of this technique is now being questioned.
Two researchers (Kalousek and Dill, University of British Columbia) have published a paper indicating that chromosomal abnormalities can be present in the placenta but absent in the child. (Science, Volume 221, August 12 1983) In other words, abnormal cells detected by chorionic sampling do not necessarily reflect the state of the child.
The authors also point out that amniocentesis can be just as inaccurate. Several investigators have reported finding chromosomal mosaicism (that is, a mixture of cells with normal and abnormal chromosome complements) in cultures of amniotic fluid that could not be confirmed in subsequently aborted fetuses or live born infants. They suggested that the cells sampled were in fact chorionic ones that as can happen, were somehow mixed in the amniotic fluid, when it was sampled for testing.
It is no surprise that chromosomal conditions can vary between the placenta and the body of the child. Of the many cells in the very early embryo (blastocyst stage), only three or four develop into the child himself; all the rest develop into the placenta.
Each time a cell divides (a process called “mitosis”), the chromosomes, which carry genetic information, must be distributed correctly. Occasionally during mitosis, accidents happen, so that, where one daughter-cell receives an extra chromosome, the other daughter-cell will be missing that chromosome. The placenta might very well be descended from such cells, and yet the child’s chromosome complement would still be normal. Much less commonly, the reverse might occur: a genetically-handicapped child might have a chromosomally-normal placenta. Either way, the sampling would give inaccurate results.
Other flaws of chorionic villi sampling have become apparent as it has been used. In a recent note to the New England Journal of Medicine (May 24 1984), Hecht, Hecht and Bixenman (Southwest, Biomedical Research Institute) stated that “… the fetal- loss rate averages 12 per cent. This is far above the rate with midtrimester amniocenesis.” They go on to point out that the risk of chromosomal abnormality even in a child having a 45 year old mother is only 5 percent. It should be added that no one knows the spontaneous-miscarriage rate for the first trimester-which is when villi sampling is done.
Concerned with hunting
In light both of the possibility of error and of the widespread concern over miscarriages, why is chorionic villi sampling so heavily promoted? Why are the risks not being made known to doctors being taught the technique? And why isn’t the current extensive publicity even raising this issue?
Perhaps those involved in promoting it are so concerned with hunting down and killing handicapped babies that they are unwilling to apply even the caution normal when introducing a new procedure.
Charles Brockhouse, M.Sc. is a graduate student in Molecular and Cytogentics at the University of Toronto.