Will Down syndrome disappear?

There are concerns about the impact of the improving accuracy and availability of low risk cheap prenatal tests such as for Down syndrome (DS). Introduction of a noninvasive maternal serum test is expected that might provide a definitive diagnosis of DS in the first trimester at no risk to the fetus. The authors report that the tests should be virtually universally available and allow privacy of decision making. The authors ask whether the new tests will decrease the birth incidence of DS even further. Indeed, might there be no more DS children born? If so, is that a problem?

Fetal DS can probably be identified by isolating cell-free fetal DNA or RNA in the maternal serum.  Genes expressed exclusively on chromosome 21 and specific to the fetus are identified. Subtle genetic variations between maternally and paternally inherited alleles are exploited to determine the number of fetal chromosomes 21.

Current tests plus elective abortion have already reversed the massive increase in DS that would have come from the aging of the obstetric population in western countries. What might be the effects of the new tests?

DS is unlikely to disappear. Not everyone has testing and not everyone chooses to terminate a pregnancy with a DS fetus. Some refuse on religious and other grounds. And tests are not available to everybody: some women present later in pregnancy; others, often have limited access, such as poor and rural women (Coory M et al.Antenatal care implications of population-based trends in Down syndrome birth rates by rurality and antenatal care provider, Queensland, 1990–2004 MJA 2007; 186 (5): 230-234).

In addition, tests are imperfect – most prenatal tests do not detect all affected fetuses. Current DS screening tests detect up to 90%. Time will tell the accuracy of new tests. Preliminary studies have predicted a definitive diagnosis will be possible in up to 95% of fetuses with near 100% sensitivity and specificity (Yang Y et al. Identification of mRNA-SNP markers for a noninvasive prenatal trisomy 21 (T21) test. Prenat Diagn 2008;28:S12).

Disability in general will not disappear no matter how sophisticated future tests are. Many abnormalities are not detectable prenatally, such as cerebral palsy and most causes of intellectual handicap. Testing will improve but will remain limited. Beautiful images of fetal structure can be obtained with ultrasound, for example, but disorders of function cause more handicap than disorders of structure. Limbs may be perfectly formed but work poorly or not at all. We have a limited ability to assess function.

Apparently minor fetal abnormalities may have serious underlying causes that can be impossible to diagnose. Mild cerebral ventricular dilatation is a relatively easy diagnosis with ultrasound but it does not cause intellectual handicap. Most children with this finding will be normal or have potentially treatable less serious delays. But uncommonly it is a marker of serious brain disorder; we have limited ability to distinguish these serious problems.

Would it be a problem if DS did disappear? It is said that parents who have children with Down syndrome have found much richness in life with an extra chromosome (Skotko B, Kidder C. Common Threads: Celebrating Life with Down Syndrome. Rochester Hills, MI: Band of Angels Press; 2001).

Parents of DS children rightly highlight their happy disposition. But few people choose to have a Down syndrome child. It is hard to find adoptive parents for a DS child. Couples with a previous DS child usually have prenatal testing to minimize the risk of having another disabled child and they proceed to abortion of an affected fetus.

Parents of disabled children, including those with DS, commonly ask: ‘How could you offer abortion for DS (or some other abnormality)? Look at our happy DS child and see how much she has enhanced ours and others lives’. But this is the wrong question. My wife and I have 4 healthy children. I did not want a fourth child (but eventually agreed to have one). He has to date had a happy and fulfilling life and has contributed much to many other’s lives; I love him dearly and our family’s lives would have been much poorer without him. But I do not feel all families with 3 children should have a fourth. Many would be wiser to limit to 3 with contraception or, if necessary, abortion. The fact that we have a loved fourth child does not mean that abortion is wrong for others who conceive a fourth.

Prenatal testing is enthusiastically embraced by pregnant women. Earlier and better tests are important to them. There is no place for doctors to deny women improved tests thereby resulting in some having a child with an abnormality that was detectable. Prenatal tests are a woman’s choice. DS children should not be imposed on unwilling parents because some have the view that it important to have such children in the world.

We should celebrate the coming of risk free early prenatal tests. Current diagnostic tests increase the risk of miscarriage. Even more importantly but less often discussed, complications of tests can result in life long deficits for the child to be. If invasive tests are seldom used, such procedure related complications may become rare.