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Mothering › Pregnancy Articles › Diagnostic Tests of the Third Trimester

Diagnostic Tests of the Third Trimester

pregnant woman getting blood drawnThe 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?




Testing for preeclampsia:
Preeclampsia, also called toxemia, is a condition that affects 5 to 10 percent of pregnancies, usually in the third trimester. Preeclampsia is pregnancy-induced hypertension characterized by elevated blood pressure, excessive retention of fluids, especially in the hands and face, rapid weight gain, double or blurred vision, growth retardation in the fetus, and protein in the urine.

Having any of these symptoms does not necessarily indicate preeclampsia, however. As well, some women with preeclampsia have no symptoms at all other than a feeling that something is not right.

No one knows what causes preeclampsia. In fact, physicians often refer to it as the “disease of theories.” You are more likely to have it if you have diabetes, are less than five-three in height, or have had a previous history of high blood pressure. The risk is also higher for first-time pregnancies and for women who have second pregnancies spaced far apart from their first.

Promising new research suggests that preeclampsia may be the result of the release of proteins into the mother’s bloodstream by the fetus. This is done, according to the theory, because the fetus is receiving inadequate amounts of oxygen and nutrients from maternal blood vessels that have not expanded enough to increase necessary blood volume to the baby.  Some studies have shown that women with vascular problems, which they may not have even known about ahead of time, are more likely to develop preeclampsia. There may also be a genetic component.

It is hoped that new diagnostic tests and medications will soon be developed to catch, and treat, this illusive disease.

If left untreated, preeclampsia can develop into the more severe eclampsia, characterized by convulsions, coma, and even maternal or fetal death. This is very rare, however, because most women receive treatment before it reaches this stage. In the more severe stages, symptoms can include abdominal pain, severe headaches, nausea, vomiting, and convulsions, and premature labor.

Preeclampsia is usually diagnosed during routine urine examinations that screen for high levels of protein. If is determined that you have this condition, you may be advised to get bed rest for most of your pregnancy, either at home or in a hospital.

There is not, as of yet, conclusive evidence that complete bed rest is helpful for women with mild to moderate preeclampsia, and it is not an easy thing to do. You will need to decide if this is right for you. You will most likely be given antihypertensive medication to keep your blood pressure down.

Testing for rhesus incompatibility: Rhesus incompatibility occurs when the mother’s blood is Rh-negative and the fetus has inherited Rh-positive blood from the father.

If fetal blood enters the mother’s bloodstream (for example, during delivery, miscarriage, abortion, amniocentesis, or fetal blood sampling) at some earlier date, her body will then produce antibodies to fight it. Although this condition rarely presents a problem in first pregnancies, the antibodies remain in the mother’s blood and in subsequent pregnancies may attempt to destroy the blood supply of the new fetus. This can result in severe anemia, jaundice, brain damage, or death to the baby.

Your blood will be tested in early pregnancy to determine if you are Rh-negative. If you are Rh-negative and your partner is Rh-positive, you will be tested frequently throughout pregnancy to determine if you have developed any antibodies. During your third trimester or soon after deliver, you will be given a vaccine (known as Rhogam) to prevent antibodies from developing and endangering future pregnancies.

In the past, Rhogam, like many vaccines, contained a preservative known as thimerosol, or mercury. The safety of this has now been questioned, and the use of mercury is being phased out, but it is worth checking with your health care provider about this concern to be certain you are receiving a mercury-free vaccine.

Comments (1)

My doctor has me scheduled for the group b strep test at 27.5 weeks instead of 37 weeks. That a large difference. Is this ok or should it really be done later?
Mothering › Pregnancy Articles › Diagnostic Tests of the Third Trimester