Diagnostic Tests of the Third Trimester

The subject of testing in pregnancy is one that often causes women anxiety, sometimes in ways they are not even consciously aware of. The information that follows is provided because many health practitioners emphasize testing in pregnancy, and therefore it is helpful for women to know the facts about these tests. As always, though, it is best to keep reminding yourself that the conditions these tests screen for are rare, and even when the conditions are present, they are treatable. Maintaining your own serenity is the most important gift of health that you can give yourself and your baby.

Testing for glucose tolerance: Throughout your pregnancy, your urine has most likely been tested for the presence of sugar. If high sugar levels are detected in your urine, you may be given follow-up tests to determine whether you are at risk for gestational diabetes, a temporary condition that will disappear after the baby is born. This should not be confused with a preexisting diabetic condition, which requires close medical supervision throughout pregnancy to prevent stillbirth and congenital defects.

Some practitioners believe that gestational diabetes can increase the risk of fetal macrosomia, or a larger-than-average baby. To prevent the baby from growing too large, and to avoid the need for cesarean, women deemed at risk for gestational diabetes are often placed under careful dietary restrictions, monitored using blood tests and ultrasound scans, and occasionally given insulin. In some cases labor is induced early, or elective cesarean is performed.

Gestational diabetes has been the cause of a great deal of controversy. Many birth practitioners believe that elevated blood sugar levels actually represent a healthy response to the inherent dynamic of pregnancy. Glucose may remain in the blood for longer periods so that it is more accessible to the developing baby. These experts believe that the pregnant metabolic state is not being taken into account correctly.

Only 30 percent of women with an abnormal glucose tolerance test will have larger than average babies. In fact, most large babies will be born to mothers with normal glucose tests. The glucose tolerance tests themselves are unreliable and can only be duplicated 30 to 50 percent of the time.

No controlled studies have proven that high blood sugar leads to problematic or high-risk pregnancy. In fact, one clinical trial that studied women with gestational diabetes found that the outcomes of elective C-sections due to larger babies showed a significantly higher incidence of infant mortality, with no better outcomes than the control group (Murray Enkins et al., A Guide to Effective Care in Pregnancy and Childbirth (New York: Oxford University Press, 2000), p. 104).

Testing for Group B streptococcus (GBS): Unlike the bacteria that cause strep throat, GBS causes blood and skin infections and pneumonia. In pregnant women, GBS can cause bladder and womb infections and stillbirth. Before prevention methods began to be used, about 8,000 babies in the United States contracted GBS infections every year, and about 5 percent died.

Between 5 and 35 percent of pregnant women temporarily carry GBS in their bodies, often in the vagina or rectum, although they do not develop symptoms. When a baby moves down the birth canal, it can become infected by the bacteria, which can lead to pneumonia, sepsis (infections of the blood or tissues), or meningitis. As infants don’t handle infection well, the disease can spread quickly, and possibly become fatal before it is even detected.

It is now routine practice to test pregnant women for the presence of GBS in late pregnancy, around week 37. A culture is taken by swabbing the rectum and vagina. Results are available a few days later.

A positive test (indicating that you have GBS) does not necessarily mean that you will transmit it to your baby. It is more likely if these risk factors exist:

• The presence of fever during labor
• A urinary tract infection due to GBS
• Rupture of membranes (water breaking) 18 hours or more prior to delivery
• Rupture of membranes prior to week 37
• A previous baby with GBS

If you test positive for GBS, antibiotics will be offered to you intravenously during labor. Taking a course of antibiotics prior to labor, however, does not protect the baby. If you decide to have intravenous antibiotic treatment, ask for a heparin lock on your intravenous line, which allows it to be unhooked so you have freedom to get up and move around during labor.

For more information about GBS, read “Treating Group B Strep: Are Antibiotics Necessary?”