The Economics of the Amniocentesis

Photo: kaatje85

An amniocentesis (or an “amnio”) is a fairly common procedure among pregnant women that involves the extraction of a small sample of amniotic fluid surrounding a fetus. The main benefit of an amnio is that it can diagnose genetic disorders in a fetus, including Down syndrome. But there is also a real cost, as roughly 1 in 200 tests causes a spontaneous miscarriage (estimates of this probability vary).

Thus the world of amnios is a world of costs and benefits — and hence amenable to economic analysis.

At least that is the argument made by Eduardo Fajnzylber, Joseph Hotz, and Seth Sanders in a provocative working paper titled “A Dynamic Model of Amniocentesis Choice.”

Consider a potential mother trying to decide: Should I get an amnio? Historically, obstetricians recommended that pregnant women age 35 or older get an amnio (or a related procedure called chorionic villus sampling).

The logic of this recommendation is superficially appealing: The rate of fetal genetic disorders is higher for older women, which suggests that they also get a greater benefit from the test. And the cost — potential miscarriage — doesn’t really change with age. Why is age 35 the cutoff point? That’s around the age at which the risk of miscarriage equals the rate at which abnormalities are successfully detected.

Other economists have pointed to the illogic of this decision rule: It only makes sense to equate the marginal probability of good and bad outcomes if the bad outcome is as costly as the good outcome is beneficial. But is this true? Is learning about a fetal disorder as beneficial as a possible miscarriage is costly? This depends on your value system.

The information revealed by an amnio won’t change a committed anti-abortionist’s mind, and hence an amnio has little benefit. For someone fearful of a child with Down syndrome, an amnio may be worthwhile despite the miscarriage risk.

But there’s even more to it than that. As you might expect, women become more likely to get an amnio as they age, particularly when they turn 35 — but only up to a point. In particular Fajnzylber, Hotz, and Sanders note that women in their 40′s are less likely to get an amnio than women in their late 30′s.

So who is more rational, the docs suggesting these older women get amnios or the women refusing them?

The economists conjecture that these women may be rationally weighing costs and benefits in a more sophisticated manner than the doctors who are giving them advice. While the cost of an amnio measured in physical units (lost pregnancies) doesn’t change with age, that’s only a partial accounting. In particular, if the amnio causes a miscarriage to a woman in her 20′s or early 30′s, she is very likely to get pregnant again and replace a miscarried fetus with a healthy fetus. (Economists call the valuation of this future possibility “option value.”)

But a woman in her late 30′s or 40′s may find it difficult to get pregnant again, and so an amnio may involve the risk of losing both a specific fetus (which was probably healthy) and also the opportunity to raise any healthy baby. In this broader accounting, the cost of an amnio-induced miscarriage rises with age.

Yes, this is a subtle and complex argument. Once you factor in this option value, both the costs and benefits of an amnio rise with age. Thus, the key issue in giving the right amnio advice is the rate at which genetic disorders increase with age, relative to decrease in the likelihood of getting pregnant again should the test cause a miscarriage to occur.

What are the broader implications of all of this? If the authors are correct that too many women in their 40′s have been advised to have amnios (because doctors aren’t thinking about the difficulty of replacing a viable fetus with a subsequent pregnancy), then the same logic may suggest that too few younger women are being advised to have amnios (because their docs aren’t thinking about the relative ease with which they can get pregnant again).

I’ll quote the authors on this: “Fully informed of the full costs and benefits of amniocentesis, some younger women would choose to replace their chromosomally abnormal fetus by a future pregnancy.” Indeed, around 80 percent of Down syndrome children are born to women under the age of 35.

As the authors admit, there’s a lot of work to be done in sorting all of these issues out. But don’t be distracted by the fact that economists rather than doctors are thinking clearly about medical costs and benefits. This is just a particularly important example of how thinking hard about basic economic principles — in this case option value — can yield unexpected implications.

The good news is that the American College of Obstetricians and Gynecologists appears to be catching up; in 2007 it issued new recommendations that “All pregnant women, regardless of their age, should be offered screening for Down syndrome.”

But do they really understand the true costs and benefits of an amnio? You can read the advice given to patients for yourself. They still emphasize the greater risk of genetic disorders for older women, but they unfortunately stay silent about the issues raised by the three economists.