Although Connecticut was sweltering through one of the hottest springs on record, Tierney continued jogging to stay in something vaguely resembling her usual shape. She ran a route that took her from their apartment building on Imlay Street through their Asylum Hill neighborhood, past the Taco Bell, the halfway houses, and the little ethnic markets located hard against Interstate 84. She also maintained a rigorous weight-training program at her gym, and it looked as though she would be the first person to stick with the regimen long enough to be used in the data the gym trainer was collecting for a Ph.D. dissertation.
All the medical indicators had been equally good. In June, as a routine follow-up to the April ultrasound test, Tierney underwent a prenatal examination called a triple screen. The test, usually performed between the fifteenth and eighteenth weeks of pregnancy, is used to check the likelihood of several kinds of major birth defects, including neural tube and ventral wall defects, in which a fetus has an abnormal opening of the spine or abdominal wall. It's also used to screen for Down syndrome, the most common of all chromosomal abnormalities, a condition marked by mental retardation and signature facial features and often accompanied by various health problems.
Introduced in 1988, the triple screen checks a sample of a pregnant woman's blood for the levels of three substances: alpha-fetoprotein (AFP), estriol, and human chorionic gonadotropin (hCG). AFP is a protein produced in the yolk sac of a pregnant woman and also by the liver of her fetus. Estriol and hCG are hormones produced by the placenta. The e.p.t. pregnancy test that Tierney had used was designed to detect hCG in urine — its presence at certain levels is a sure sign of pregnancy. In fact, hCG was the substance used in the old "rabbit tests." Doctors in the 1920s discovered that rabbits injected with urine rich in hCG — the urine of pregnant women, that is — would undergo distinct changes to their ovaries. Doctors would kill the rabbits to see if those changes had taken place. Although the rabbit died regardless of whether a woman was pregnant, the phrase "The rabbit died!" became mistakenly synonymous with a positive result.
The triple screen, then, is a sophisticated rabbit test. Scientists discovered that elevated or lowered levels of the three substances at specific points in a pregnancy can suggest a host of potential problems. Proper levels suggest the absence of those problems — in other words, a healthy child. For instance, a high level of AFP at certain times suggests a greater likelihood of a neural tube defect such as spina bifida, a physical deformity that results from the failure of the spine to close properly during the first month of pregnancy. Lower-than-expected levels of AFP and estriol, combined with higher-than-expected levels of hCG, can mean Down syndrome.
There's little dispute about the value of the triple screen, but it's far from perfect. By their very nature, so-called screening tests such as the triple screen and ultrasound provide good indications of what's happening inside a womb, but no definitive answers. It's as though they hear thunder and automatically predict rain: the two usually go together, but not always. Screening tests are known for false negatives, where bad news masquerades as good. For instance, the triple screen only picks up about 60 percent to 70 percent of instances of Down syndrome.