Pregnancy: Non-stress test

I am 41 weeks pregnant and my midwife has ordered a non-stress test. Since I'm not even two weeks overdue yet, is this really necessary? Please tell me more about this test.

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Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

A pregnancy does not formally become "post-term" until completion of the 42nd week of pregnancy. The risk of stillbirth increases significantly enough after 41.5 weeks so that "antepartal fetal testing" has become the standard of care at this time.

Some protocols dictate that such testing begin at 41 weeks, some at 41.5 weeks and some begin this only at 42 weeks. The purpose of antepartal fetal testing is to determine the well-being of a fetus at risk for antepartal fetal loss.

When the NST is "reactive" or "normal," research has shown that in most cases, it is unlikely that the baby will die in utero within the next five days. This is why the test should be repeated every four to five days until birth.

No test is perfect, however, and there are many false positives associated with such testing. This means that if you had a "nonreactive" NST, it would not necessarily indicate that your baby was in distress. Rather it is an indication for further investigation and testing.

Research that led to the development of antepartal testing demonstrated, that when a baby has insufficient oxygen or "uteroplacental insufficiency," the fetal heart beat will have decreased "variability." This means that it will remain constant and not change with circumstances. It will not adapt for example to fetal movement, staying static. In a baby who is not stressed by hypoxia (decreased oxygen), the heart rate should accelerate with fetal movements.

The non-stress test is performed with the patient reclining and is best performed soon after a meal. An external fetal monitor (belts applied to the top and lower part of the maternal abdomen) records fetal movement and the response of the fetal heart. A "reactive" test is one that shows two 15 beat accelerations of the fetal heart rate accompanying movement in a 20 minute period of time. Each acceleration must last 15 seconds. A "nonreactive" test is one that shows no accelerations in a 40 minute period of time or fails to meet the requirements of a reactive test.

Fetal sleep may impede the NST so it may be repeated after 40 to 60 minutes if it is nonreactive. It never should be left to be repeated the next day.

As with all surveillance testing, the NST is not perfect. It misses some babies who would benefit from more careful monitoring and it raises concerns for some babies who are perfectly healthy. Other parameters such as maternal indices of well-being and observations of fetal movement also have their place in the assessment of the mother and baby.

A next logical step after a nonreactive NST is the contraction stress test (CST). A very dilute solution of oxytocin is administered via intravenous tubing and contractions begin to appear on the fetal monitor. These contractions may not even be noticeable to the mother and are not enough to induce labor. When the mother has three contractions in ten minutes and the fetal heart shows no decelerations, the test is considered negative.

Postmaturity is the most common reason to schedule such tests. I would not question the wisdom of performing the NST at 41 weeks. Your midwife wants to reassure everyone that the baby is well despite his or her delayed entry into the world.

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