Non-stress test; NST; CST; Contraction; Fetal heart monitoring
How the test is performed
EXTERNAL FETAL MONITORING
By definition, external fetal monitoring is done through the skin (transdermally) and is not meant to be invasive. You will sit with knees and back partially elevated with a cushion under the right hip, which moves your uterus to the left, or however you are comfortable, as long as your uterus is displaced to the left or for brief periods on the right.
Sensitive electrodes (monitors) are placed on your abdomen over conducting jelly that can sense both fetal heart rate (FHR) and the strength and duration of uterine contractions. Usually, the output or results of this test are continuous and are printed out, or appear on a computer screen.
This allows your health care provider to monitor if your baby is experiencing fetal distress, and how well the fetus is tolerating the contractions. The decision to move to internal fetal monitoring is based on the information first obtained by external fetal monitoring.
NON-STRESS TEST (NST)
The NST is another way of externally monitoring your baby. The NST can be done as early as the 27th week of pregnancy, and it measures the FHR accelerations with normal movement. For this test, you will sit with knees and back partially elevated with a cushion under the right hip, which moves your uterus to the left.
The same monitors described above are placed on your abdomen to measure the ability of the uterus to contract and the FHR. If there is no activity after 30-40 minutes, you will be given something to drink or a small meal which may stimulate fetal activity. Other interventions that might encourage fetal movement include the use of fetal acoustic stimulation (sending sounds to the fetus) and gently placing your hands on your abdomen and moving the fetus.
CONTRACTION STRESS TEST (CST)
The CST is a final method of externally monitoring your baby. This test measures the ability of the placenta to adequately oxygenate the fetus under pressure (contractions).
For this test, you will sit with knees and back partially elevated with a cushion under the right hip, which moves your uterus to the left. The same monitors described above are placed on your abdomen to measure uterine contractility and FHR. If contractions are not occurring spontaneously, either a medication (called oxytocin) will be given intravenously, or nipple stimulation will be used to induce contractions.
If oxytocin is administered, it is called the oxytocin challenge test (OCT). Oxytocin is administered through an IV until three uterine contractions are observed, lasting 40 to 60 seconds, over a 10-minute period.
A test involving nipple stimulation is called the nipple stimulation contractions stress test. Every effort will be taken to ensure your privacy, but the nurse will be with you through the entire procedure.
After being positioned as described above, you will rub the palm of your hand across one nipple through your garments for 2 to 3 minutes. After a 5-minute rest, the nipple stimulation should continue until 40 minutes have elapsed, or 3 contractions have occurred, lasting more than 40 seconds, within a 10-minute period. If a uterine contraction starts, you should stop the nipple stimulation.
INTERNAL FETAL MONITORING
Internal fetal monitoring involves placing a electrode directly on the fetal scalp through the cervix. Your health care provider may use this method of monitoring your baby if external monitoring is not working well, or the information is suspicious.
A vaginal examination will be performed, and the electrode will be introduced with its plastic sheath into the vaginal canal. This plastic guide is moved through the cervix and placed on the fetus’ scalp, then removed. The electrode’s wire is strapped to your thigh, and attached to the monitor.
How to prepare for the test
An explanation of the procedure and its risks is provided by your health care provider. You will be asked to wear a hospital gown and sign a consent form prior to the procedure.
How the test will feel
External fetal monitoring:
Sitting in place for extended periods of time can become uncomfortable for some people. If this is the case, your health care provider (once notified) will help reposition you to a more comfortable position.
The jelly that is placed under the external monitors is the same used for ultrasounds, and may be cold.
Internal fetal monitoring:
Some patients report feeling mild discomfort while the electrode is inserted through the cervix.
Why the test is performed
Both types of tests are performed to evaluate fetal heart rate and variability between beats, especially in relation to uterine contractions. The tests also indicate the frequency and strength of uterine contractions.
This information is invaluable in determining how well your baby is tolerating the birth process, and if there needs to be emergency intervention.
Normal values indicate that the fetus is not in distress by showing a fetal heart rate between 120 and 160 beats per minute, with variability of 5-25 beats per minute from the baseline (normal) fetal heart rate.
It is not uncommon for the fetal heart rate to drop slightly during a contraction, since placental blood supply is diminished under the compression of a uterine contraction, as long as the FHR recovers quickly once the contraction has stopped.
What abnormal results mean
The following situations or conditions are considered abnormal results, and may be detected by your health care provider (by monitoring the data from these tests):
- Cord compression (there is no free blood flow to the fetus)
- Fetal heart block (where there is a block of electrical flow within the heart muscle causing an altered heart rhythm)
- Fetal malposition
- Fetal hypoxia (insufficient oxygen supply to the fetus)
- Infection (monitoring cannot diagnose an infection, but can suggest the presence of an infection)
- Uteroplacental insufficiency (insufficient oxygen exchange between the uterus and the placenta)
- Fetal distress
- Abruptio placenta
What the risks are
External fetal monitoring:
There are no risks associated with external monitoring.
Internal fetal monitoring:
- Fetal scalp bruising
by Sharon M. Smith, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.