Antepartum testing involves the use of electronic fetal monitoring (EFM) or ultrasound (US) to assess fetal well-being as determined by the fetal heart rate (FHR) and other characteristics during the antepartal period, which is the period spanning from conception to labor. Antepartum tests include the nonstress test (NST), modified biophysical profile (MBPP), contraction stress test (CST), oxytocin contraction stress test (OCT), biophysical profile (BPP), doppler flow studies, amniocentesis , cordocentesis, and fetal echocardiography.
Antepartum testing can start as early as 24 weeks but usually begins after 32 weeks of pregnancy depending on the mother's physical status. It provides a means for the physician and pregnant woman to identify any problems and be alert to any changes that may necessitate additional testing or interventions. The testing results reflect how well the placenta is functioning in its ability to adequately supply blood and, therefore, oxygen to the fetus.
The testing is done for pregnancies at risk for maternal and/or fetal complications. Some of these risks include:
- any chronic illness in the mother, such as high blood pressure, diabetes, or autoimmune diseases, including systemic lupus erythematosus (SLE)
- problems with previous pregnancies, such as a history of unexplained stillbirth
- fetal complications, such as intrauterine growth restriction (IUGR) (growth in the fetus below the tenth percentile), birth defects, twins , or other multiple gestations in which a growth discrepancy occurs, such as twin-to-twin transfusion syndrome
- problems in current pregnancy, including pregnancy-induced hypertension frequently referred to as preeclampsia); gestational diabetes (diabetes caused by pregnancy) requiring the use of insulin; premature rupture of the membranes (PROM); too little or too much amniotic fluid (the liquid surrounding the fetus) called oligohydramnios and polyhydramnios, respectively; vaginal bleeding; placental abnormalities, i.e., partial abruption (a portion of the placenta pulls away from the wall of the uterus), or placenta previa (a condition in which the placenta is covering the cervix instead of near the top of the uterus)
Antepartum testing is also used in low-risk pregnancies to evaluate decreased fetal activity, a lag in fundal height (as measured from top of the pubic area to the highest point in the midline at the top of the uterus), and postdates or post-term pregnancy. A normal pregnancy is 40 weeks and testing should begin at 41 weeks to assess the status of the placenta, which may no longer be capable of meeting the baby's needs. This can be indicated by the FHR pattern, amniotic fluid status, and fetal movement patterns.
The spectrum of fetal assessment includes fetal movement (FM) counting, nonstress test (NST), modified biophysical profile (MBPP), contraction stress test (CST), oxytocin contraction stress test (OCT), biophysical profile (BPP), doppler flow studies, amniocentesis, cordocentesis, and fetal echocardiography. Fetal movement should be determined on a daily basis by all pregnant women regardless of risk status. The woman should be instructed to monitor fetal movement by selecting a consistent time of day to document how long it takes to feel 10 fetal movements. She should call her healthcare provider if there are fewer than 10 movements in a 10-hour period or immediately if there are no movements in any 10-hour period. She should also be instructed to report significant decreases in fetal activity from the baby's normal pattern. This daily monitoring of FM by all pregnant women is the least expensive and easiest of all antepartum tests to perform.
Non-stress test (NST)
The NST is performed with an electronic fetal monitor (EFM) that traces the fetal heart rate (FHR) and the presence of any contractions on a monitor strip. The mother reclines with a slight pelvic tilt to prevent compression of the large blood vessels by the pregnant uterus. The EFM is applied to her abdomen by two straps: one is to listen to the FHR by means of a transducer, and the other is to pick up any contractions by pressure on a tocodynameter. The NST indirectly provides information about fetal status by the observation of FHR accelerations that occur with fetal movement. If a fetus is not receiving adequate oxygen from the placenta, the FHR will not accelerate, but if the oxygen supply is sufficient, accelerations will be noted. If it is difficult to obtain fetal movements, a vibroacoustic stimulator (VAS) is sometimes used to provide a loud noise to awaken the fetus and produce the desired results. The minimum amount of time required for an NST is 20 minutes. During those 20 minutes, there must be two accelerations in the FHR that are 15 beats above the baseline FHR and last for 15 seconds, often called the 15 by 15 rule. Depending upon the conditions, however, it may sometimes take up to 60 to 90 minutes to obtain definitive results.
Modified biophysical profile (MBPP)
The MBPP is performed in the same manner as the NST with a limited ultrasound (US) performed to assess the amount of amniotic fluid, which is reported as the amniotic fluid index (AFI). Following the NST, an US is done to observe the amount of amniotic fluid present in four quadrants, which are divided along the umbilicus midline and perpendicular to the midline. There must be no fetal parts or any umbilical cord present in any of these pockets of fluid in order to be counted.
Contraction stress test (CST) and oxytocin contraction stress test (OCT)
The contraction stress test (CST) is similar to the NST except the FHR is evaluated for accelerations, 15 beats higher than baseline lasting 15 seconds, and in response to contractions as well. A CST requires the presence of three uterine contractions (UCs) within a 10-minute period lasting at least 40 seconds and of moderate intensity. During a contraction, the blood flow to the baby is temporarily restricted, which provides a form of "stress" to the baby. The baby's response to this stressor reveals significant information regarding available oxygen stores. If contractions are not spontaneously present, the pregnant woman will be instructed on the use of nipple stimulation to produce contractions through the release of natural oxytocin, or oxytocin can be administered through an intravenous infusion (IV) called pitocin to produce contractions. When oxytocin is administered IV, it is called an oxytocin contraction stress test (OCT). The CST/OCT is generally used after an abnormal NST is obtained in order to verify if there are problems present. Many clinicians require their diabetic patients to have at least one CST/OCT a week to assure fetal well-being. Maternal blood pressure is taken with each test.
Biophysical profile (BPP)
The biophysical profile (BPP) is performed by an ultrasound exam over a 30-minute period. The ultrasonographer/examiner looks for gross fetal movement, i.e., kicking and moving around; fetal tone, i.e., making a fist; breathing movements (which the mother can often perceive as hiccoughs); and amniotic fluid volume. A score of 0 or 2 points is assigned to each observation with the results of the NST also adding 2 points for a total possible score of 10 points.
Doppler flow studies, amniocentesis, cordocentesis, and fetal echocardiography
A physician or specially trained ultrasonographer performs Doppler flow studies, which examines the blood flow in the umbilical artery and the baby's middle cerebral artery. An experienced obstetrician/perinatologist performs the amniocentesis or the cordocentesis. For the amniocentesis, an US is used to determine an appropriate place to insert a needle and withdraw amniotic fluid for testing. In a similar manner, US is used with cordocentesis, but in this procedure a needle is guided into the umbilical cord to withdraw fetal blood for testing. Fetal echocardiography is a specialized ultrasound of a baby's heart. Since it detects most congenital heart defects, it is recommended if a baby is at a higher risk for a defect than the risk in the general population. The majority of health insurance companies do cover a portion, if not all, of the tests' costs.
Clinicians should only prescribe these tests if they are ready to intervene when faced with worrying results. A fetus is considered viable at 24 weeks since that is the minimum gestational age for sufficient lung development. There are no significant risks to the mother or the fetus from the nonstress test (NST), modified biophysical profile (MBPP), or the biophysical profile (BPP). Ultrasound waves utilized in detecting the FHR and for the BPP are painless and safe because this method employs no harmful radiation. There is no evidence that sound waves cause any harm to the mother or the baby.
If the test results are acceptable, the pregnant woman is instructed to continue following her current medical regimen and return for additional testing on the dates prescribed. For NSTs/MBPPs/CSTs, the time period between tests should be no longer than three to four days under high-risk conditions with fetal movement counting taking place in between testing dates. Ultrasounds should be rescheduled as the need dictates per the physician. Electronic fetal monitoring occurs after an amniocentesis or cordocentesis to assure fetal well-being.
There are no complications per se from the tests themselves with the exception of unfavorable test results or supine (lying horizontality on the back) hypotension secondary to a pregnant woman lying on her back for an ultrasound. When a pregnant woman lies on her back, the vena cava (one of two large veins that return blood from peripheral circulation to the heart) can become compressed from the pressure of the pregnant uterus such that blood flow to the heart is significantly reduced. There are potential complications from an amniocentesis, i.e., preterm labor, spontaneous rupture of membranes, fetal or placental injury; and the clinician performing the procedure should explain what these are prior to the procedure. There are similar risks and potential complications from cordocentesis as well which should be explained.
Usually, a report of normal results for NSTs provides reassurance that the fetus is healthy and should remain so for three to four days, at which time repeat testing will be necessary. A normal NST is reported as being reactive, which means the fetal heart is "reacting" to movement such that the FHR is accelerating 15 beats per minute above the baseline FHR for 15 seconds twice within a 20-minute period of time. A non-reactive NST is one that fails to meet this criterion within an 80 to 90 minute period of time. For an extremely preterm fetus, a normal NST is reported as being reactive for gestational age, which indicates the FHR demonstrated two accelerations of 10 beats per minute above baseline for 10 seconds over a 20–30 minute period. Typically, the central nervous system is not completely mature until approximately 32 weeks gestational age, and a report of reactive for gestational age takes this into consideration. It is important to remember that a normal result does not guarantee that no problems are present. Although very rare in occurrence, false normal results can be observed.
The CST results are reported as reactive/negative, reactive/suspicious, reactive/positive (a very unlikely result), or non-reactive/negative, non-reactive/suspicious, non-reactive/positive. The reactive/non-reactive part of the test report refers to the presence or absence of accelerations. The negative part refers to no decelerations being present with uterine contractions (UCs). "Suspicious" refers to the presence of some decelerations with UCs, and "positive" refers to the presence of decelerations more than 50 percent of the time with UCs. A suspicious or positive result requires further evaluation, i.e., prolonged EFM monitoring or a BPP. A normal BPP report without an NST is 8 points and 10 points with a reactive NST. Six points is suspicious and requires either a CST or a repeat BPP within 24 hours. A total of 4 points is not reassuring and requires immediate evaluation by prolonged EFM.
All results are given to the primary physician who must then make a decision as to the appropriate course of action. Abnormal CST results generally indicate the baby is not receiving sufficient oxygen and may not be capable of withstanding the stress of labor and subsequent vaginal delivery. If this is the case, a cesarean section may be performed. The final outcome depends on the mother's individual circumstances. Severe pregnancy-induced hypertension may require immediate delivery via cesarean section. In some cases, medications such as betamethasone may be given to the mother to speed up the lung maturity of the baby. If the mother's cervix is favorable for induction, labor may be induced.
Amniotic fluid —The liquid in the amniotic sac that cushions the fetus and regulates temperature in the placental environment. Amniotic fluid also contains fetal cells.
Deceleration —A decrease in the fetal heart rate that can indicate inadequate blood flow through the placenta.
Fundal height —Measured by a tape measure from the top of the symphysis pubis, over the arch of the growing uterus, to the top of the fundus.
Oxytocin —A hormone that stimulates the uterus to contract during child birth and the breasts to release milk.
Pitocin —A synthetic hormone that produces uterine contractions.
Vibroacoustic stimulation —In the biophysical profile, use of an artificial larynx to produce a loud noise to "awaken" the fetus.
The healthcare provider should give a complete explanation to the pregnant woman and her partner regarding the tests, i.e., what to expect, how long the test may take, what it means, and why it is being done. It frequently helps if the pregnant woman has eaten prior to undergoing the test. Pregnant women should know that every test is not compulsory, and that if the results of the test do not matter to the parents, it may not have to be performed.
See also High-risk pregnancy .
Freeman, Roger, et al. Fetal Heart Rate Monitoring , 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2003.
Gabbe, Steven, et al. Obstetrics , 4th ed. London: Churchill Livingstone, 2001.
Neilson, J. P., and Z. Alfirevic. "Doppler ultrasound for fetal assessment in high risk pregnancies (Cochrane Review)." In The Cochrane Library , vol. 3. Chichester, UK: John Wiley & Sons, Ltd., 2004.
Pattison, N., and L. McCowan. "Cardiotocography for antepartum fetal assessment (Cochrane Review)." In The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd., 2004.
Tucker, Susan. Pocket Guide to Fetal Monitoring and Assessment , 4th ed. St. Louis, MO: Mosby, 2000.
American College of Obstetricians and Gynecologists. 409 12th Street, SW, PO Box 96920, Washington, DC 20090. Web site: http://www.acog.org.
Association of Women's Health, Obstetric, and Neonatal Nursing. 2000 L Street, NW, Suite 740, Washington, DC 20036. Web site: http://www.awhonn.org/.
Linda K. Bennington, RNC, MSN, CNS