Glucose Screening Test

Expectant mothers are routinely given a glucose screening test between 24 and 28 weeks. We're about to hit that point, so I did a bit of research.

Baby Center has a good overview of the test and gestational diabetes:

The reason for this test is to detect gestational diabetes ("GDM"). Gestational diabetes is the most common complication of pregnancy, occurring in between 2% and 5% of all pregnancies. If the mother develops gestational diabetes, additional glucose from her blood crosses the placental barrier to the baby. This leads to an extra-large baby, increasing the chance of needing a C-section or of having other delivery complications. Some sources say these babies are also prone to obesity and adult-onset diabetes later in life.

The test itself is simple: drink a shot of glucose in the form of flavored syrupy liquid, wait an hour, and get your blood sugar tested. If it's abnormally high, meaning that your body is doing a poor job of breaking down the glucose, you'll be scheduled for a glucose tolerance test.

20% of glucose screening tests are far enough out of the normal range to demand a glucose tolerance test. Only 15% of those glucose tolerance tests reveal gestational diabetes. (That would indicate a prevalence of 20% x 15%, or 3%, overall, which is consistent with their other numbers).

I found a 1998 paper on Screening for Gestational Diabetes Mellitus (from the proceedings of an international conference on GDM). Among the points the author makes:

GDM is a statistically-defined condition, not a disease.
Although there are risk factors that make a woman more likely to develop GDM (including a prior baby over 9 pounds or stillborn, family history of diabetes, obesity, and hypertension), several studies have shown that these risk factors are present in only half of the women who develop GDM. "That means that if risk factors alone determined who was to be screened, one-half of all cases of GDM would not be detected."
It's important to know what level of plasma glucose your physician is testing for. "The 140 mg/dl threshold currently recommended by the American College of Obstetricians and Gynecologists (6) yields a 79% sensitivity and results in ~6% of screened gravidae being referred for diagnostic testing. Decreasing the threshold to 130 mg/dl increases the sensitivity of the screening test to 100%, but it results in 15–20% of the screened population being referred for diagnostic testing." In other words, there's a tradeoff between catching everyone and having lots of false positives on the screening tests.
Studies where the screening was repeated on the same woman on successive days have shown that between 1/8 and 1/3 of pregnant women test as normal on one day and at risk on the next, or vice versa.

The American Diabetes Association does not recommend universal screening. The reason they give for this is that some women are at a lower risk, "and it is likely not cost-effective to screen such patients" [personal aside: I don't agree with referring to mothers-to-be as "patients"]. They recommend screening all women over the age of 25, those under the age of 25 who are obese, those with a family history of diabetes, and members of ethnic groups with a high incidence of diabetes ("Hispanic-American, Native American, Asian-Amercian, African-American, or Pacific Islander").

Most of the resistance to universal screening from organizations such as the ADA and ACOG (who also apparently recommend screening only high-risk women, but whose web site is open to members only) appears to be based either on cost/benefit grounds or on the notion that the 15% false positive rate scares too many women and subjects them to the unnecessarily annoying glucose tolerance test.

However, there are certainly more radical critiques of the glucose screening test out there. One well-written article by Henci Goer, "Gestational Diabetes: the Emperor Has No Clothes" argues that the test is useless and that the standard management practices for women who test positive are flawed and perhaps even dangerous. "The main rationale for current GD management is to reduce the incidence of birth injuries and cesarean section by reducing the incidence of macrosomia. The goal of reducing birth weight raises philosophical problems. As with glucose values, doctors are defining deviation beyond an arbitrary point as inherently pathological. Moreover, can we justify manipulating the growth mechanism of a group of babies roughly 75 percent to 80 percent of whom will fall below the 90th percentile for weight if left alone?"

The Gentlebirth Archives have a great deal of alternative information about the screening test, including comments from mothers who decided to decline the test, and physicians and midwives who only use it in specific cases. One thing they point out is that a 50-gram slug of glucose doesn't match the way that many mothers eat; if you're not snacking on sweets, you're probably not subjecting your body to such a concentrated dose of sugars to deal with. The major treatment for gestational diabetes is to eat a healthy diet, so if you're eating a healthy diet anyhow, there's no point in going through the test.