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 (ther period spanning conception and labor).
Antepartum testing can start as early as 24 weeks but usually after 32 weeks of pregnancy depending on the status of the mother. It provides a means for the physician and pregnant woman to identify fetal wellbeing and be alert to any changes that may necessitate additional testing or interventions. The results of testing reflect the functioning of the placenta and 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:
- gestational (insulin requiring) diabetes
- intrauterine growth restriction
- chronic hypertension or pregnancy induced hypertension
- too little or too much amniotic fluid (oligohydramnios and polyhydramnios, respectively)
- history of unexplained stillbirth
- autoimmune diseases, including systemic lupus erythematosus
- multiple gestation
- placental abnormalities, i.e. partial abruption (a portion of the placenta pulls away from the wall of the uterus), placenta previa (The placenta is covering the cervix and subsequent bleeding occurs.)
Antepartum testing is used in low-risk pregnancies to evaluate decreased fetal activity, a lag in fundal height (the height of the fundus, measured from pubic symphysis to the highest point in the midline at the top of the uterus), and a postdate pregnancy. A normal pregnancy is 40 weeks and testing should begin at 41 weeks to assess the status of the placenta, which may be no longer capable of meeting the baby's needs. This can be indicated by the FHR pattern, amniotic fluid status, and fetal movement patterns.
Clinicians should only prescribe these tests if they are ready to intervene when faced with ominous data. A fetus is considered viable at 24 weeks, as 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) or the biophysical profile (BPP). Ultrasound waves utilized 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.
The spectrum of fetal assessment includes fetal movement (FM) counting, nonstress test (NST), contraction stress test (CST), oxytocin contraction stress test (OCT), biophysical profile (BPP), Doppler flow studies, amniocentesis, and cordocentesis. Fetal movement can be determined on a daily basis by the pregnant woman who should be instructed to monitor fetal movement between tests by selecting a consistent time of day and documenting how long it takes to feel 10 fetal movements. She should call her health care provider if there are fewer than 10 movements in 10 hours, or 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 the mother is the least expensive and easiest of all antepartum tests to perform.
The nonstress test (NST) is performed with an electronic fetal monitoring (EFM) that traces the fetal heart rate (FHR) and the presence of any contractions on a monitor. The mother reclines with a slight tilt and the EFM is applied to her abdomen by two straps. The NST indirectly provides information about fetal status by the observation of FHR accelerations 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 observed. If it is difficult to obtain fetal movements, a vibroacoustic stimulator (VAS) may be used to provide a loud noise that will awaken the fetus and produce the desired results. The minimum amount of time required for an NST is 20 minutes, but, depending upon the conditions, it may take 60-90 minutes to obtain definitive results.
The contraction stress test (CST) is like the NST except the FHR is evaluated in response to contractions as well as for accelerations. 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 the oxygen stores available. If contractions are not present, oxytocin can be given to produce contractions or nipple stimulation may be utilized to produce contractions through the release of natural oxytocin.
The biophysical profile (BPP) is done by an ultrasound exam over a 30-minute period and the examiner looks for fetal movement, fetal tone, breathing movements (which the mother can perceive as hiccups), and amniotic fluid volume. A score of 0 or 2 points is assigned to each observation with the NST also adding 2 points for a total possible score of 10 points. The modified BPP includes an NST and the determination of the amniotic fluid index by ultrasound, which relates to the amount of fluid present. A physician performs the Doppler flow studies, amniocentesis, and cordocentesis. The majority of health insurance companies do cover a portion, if not all, of the tests' costs.
The health care provider gives a complete explanation to the pregnant woman about the test, 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.
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/CSTs, the time period between tests should be no longer three to four days under high-risk conditions. Ultrasounds should be rescheduled as the need dictates per the physician.
There are no complications per se from the tests themselves with the exception of unfavorable test results or supine (lying horizonally on the back) hypotension secondary to a pregnant woman lying on her back for an ultrasound with resultant vena cava (one of two large veins that return blood from peripheral circulation to the heart) compression.
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 means the FHR accelerated at least 15 beats above the baseline FHR for 15 seconds within a 20-minute period of time. A non-reactive NST is one that fails to meet this criterion within an 80-90-minute period of time. For an extremely preterm fetus, a normal NST refers to reactive for gestational age or the FHR accelerated 10 beats above the 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 this report takes that 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 a reactive/negative, suspicious, or positive or non-reactive/negative, suspicious, or positive. The reactive/nonreactive part of the test report refers to the presence or absence of accelerations and the negative part refers to no decelerations being present with UCs. Suspicious and positive refer to the presence of decelerations with Ucs. This result requires further evaluation, i.e. prolonged EFM monitoring or a BPP. A normal BPP report is 8-10 points. 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 usually indicate the baby is not receiving sufficient oxygen and may not be able to withstand the stress of labor and vaginal delivery. If this is the case, a cesarean section may be performed. The final outcome depends on the mother's individual circumstances, i.e. severe pregnancy induced hypertension may require immediate delivery via cesarean section, and an extra large fetus of a diabetic mother may require the same. In some cases, medications may be given to the mother for her condition, and to speed up the lung maturity of the baby. If the mother's cervix is favorable for induction, labor may be induced.
Health care team roles
The physician is the head of a health team of interdisciplinary members and determines the medical regimen necessary for the pregnant woman, depending on whether the pregnancy is normal or high-risk. Nurses schedule the tests and provide the woman with the necessary information regarding the tests. Nurses must perform any tests that they conduct correctly, interpret them appropriately, and provide the necessary follow-up or interventions. They may utilize this time period with the woman for teaching, answering questions, and offering emotional support. The ultrasonographer performs the BPP and reports the results directly to the physician.
Acceleration— An increase in the fetal heart rate that can indicate normal placental blood flow to the fetus.
Amniocentesis— A procedure by which amniotic fluid is obtained for biochemical determinations, i.e. fetal lung maturity, genetic studies.
Amniotic fluid— The liquid that surrounds the baby within the amniotic sac. It is composed mostly of fetal urine, thus, a decreased amount can indicate inadequate placental blood flow to the fetus.
Cordocentesis— A method of obtaining a fetal blood sample from the umbilical cord, also called percutaneous umbilical blood sampling (PUBS).
Deceleration— A decrease in the fetal heart rate that can indicate inadequate placental blood flow to the fetus.
Doppler flow studies— A procedure for measuring blood flow that is helpful in determining abnormalities.
Oxytocin— A natural hormone that produces uterine contractions.
Ultrasound— A procedure in which high-frequency sound waves are used to create an image of a baby. It can be used alone or with other antepartum testing.
Vibroacoustic stimulation— An artificial larynx that produces a loud noise to stimulate the fetus to wake up. It should not be used more than three times in a testing period.
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Star, Winifred L., et al. Ambulatory Obstetrics. San Francisco: UCSF Nursing Press, 1999.
Spong, Catherine L. "Antepartum Fetal Monitoring: When, What, and How." Contemporary OB/GYN Archive (September 1998). 〈http://www.perinatology.com/telemed.html〉.
Von Almen, William F. "Antepartal Fetal Surveillance." OBGYN.net Publications, 2000. 〈http://www.obgyn.net/pb/articles/surveillance.html〉.
National Perinatal Association. 3500 East Fletcher Avenue, Suite 205, Tampa, FL 33613-4712. (888) 971-3295.