Antepartum Fetal Assessment

Obstetric care has changed dramatically over the years as the fetus has become more accessible through the use of ultrasound, increased knowledge of maternal-fetal physiology and more treatment options. Perinatal Nurses are increasingly challenged to use high-technology antepartum fetal assessment testing to provide safe and sensitive care to high-risk pregnant women and their fetuses. Fetal assessment is part of the process of providing prenatal care. It involves early identification of real or potential problems and enables pregnant women to achieve the best possible obstetric outcome.

Pregnancy provides a unique opportunity to assess a women’s general health. The objective of routine antenatal laboratory tests is to recognize an unknown condition. Pregnancy can initiate dramatic changes in various systems, so routine antenatal laboratory tests help detect alterations from what is considered normal for pregnancy. Ideally, routine laboratory in the non-pregnant state would provide a baseline from which to determine the alterations that occur with pregnancy. The goal is to maintain the health of the mother in order to assure the well-being of the fetus. Frequently that luxury is not possible since many women do not seek medical care until they are already pregnant. Generally routine fetal surveillance through antepartum monitoring and testing is initiated in the event of a non-reassuring sign or when indicated by risk problems.

This module provides information on antenatal laboratory test, biophysical assessment (including ultrasound, the biophysical profile and daily fetal movement counts) and on biochemical assessment (including amniocentesis, percutaneous umbilical blood sampling and chorionic villus sampling). The procedures for non-stress and contraction stress testing and an overview of other fetal assessment parameters will be discussed.

Blood type and Rh factor – Every pregnant woman must have her blood type and Rh factor determined. Records of previous determinations have a 5% error rate. If the patient’s blood type is O and/or Rh-negative, it may be advisable to test the father of the fetus to determine if there may be Rh or ABO incompatibility if the father is Rh-positive, A, B or AB.

Antibody screen for sensitization – Every pregnant woman must have her serum screened for antibodies formed from exposure to major or minor blood group antigens. Exposure may have occurred naturally or from a transplacental hemorrhage, abortion or blood transfusion. If the screen is positive, the antibody must be identified and titered.

Hemoglobin (Hb) and hematocrit (Hct) – During pregnancy the blood volume increases by 30-50%. Since plasma volume increases more than red cell volume. Hb and Hct will fall. Mild anemia is Hb < 11 gm/dl (Hct 27-33%); severe anemia is Hb < 9 gm/dl (Hct < 27%). Anemia during pregnancy is usually of the iron-deficiency type. A folic acid deficient state may co-exist with iron deficiency. It is recommended to repeat the Hb and Hct at 28 weeks gestation.

Leukocyte count – This test is used primarily as a screen to rule out leukemia and possible infection. Normal values may reach 16,000 in pregnancy.

Differential smear – This is done primarily to identify the types of leukocytes, erythrocytes, abnormalities and the adequacy of platelets.

Serology – This test is carried out to diagnose maternal syphilis, which can affect the fetus. VDRL is a screening test.

Rubella titer – The purpose of this screening test is to determine whether the mother is susceptible to rubella (titer <1:8) or immune (titer <1:16). Rubella vaccine must not be administered during pregnancy. A few days postpartum is an excellent time to immunize a seronegative woman (titer < 1:8)

Fasting and one-hour blood sugar – Since pregnancy may precipitate glucose intolerance, it is an ideal time to screen for diabetes. The following risk factors can be used to select patients for glucose intolerance screening:

• Age > 25 years
• Family history of diabetes
• Glycosuria
• Previous infant with congenital anomalies
• Previous fetal or neonatal death
• Previous infant > 4, 000 gm
• Polyhydramnios
• Fetal size large for dates

Repeat test may be done at 28 weeks.

Alpha -Fetoprotein – Screening for neural tube defects is done at 14-18 weeks. Open neural tube defects leak α-fetoprotein through the meninges into the adjacent amniotic fluid. This produces high levels of amniotic fluid α-fetoprotein generally >5 SD above the mean. The high amniotic fluid levels of AFP are also reflected in maternal serum. False elevations may occur with erroneous dates, twins, intrauterine demise

Urinalysis must be performed on clean-catch specimens. The following tests can identify patients with asymptomatic kidney or bladder disease:

Microscopy – Microscopic examination of a centrifuged specimen can identify bacteria, leukocytes, and erthrocytes, which may indicate infection. Casts and/or red cells may indicate chronic pyelonephritis. A complete urinalysis should be repeated at 28-30 weeks.

Glucose – Glycosuria may occur in pregnancy because of increased glomerular filtration rate. However, it may also indicate carbohydrate intolerance. The test should be repeated; if positive (≥1+), further testing should be carried out. Repeat this test at each visit.

Protein – A value over 1+ is abnormal. The cause should be identified (urinary tract infection, pregnancy-induced hypertension, renal disease).

Leukocyturia – The leukocyte esterase reagent test strip is helpful in identifying patients with significant leukocyturia. Current opinion is that the test strip could reduce screening costs by replacing microscopy.

Pap smear – This identifies cancer of the cervix and cervical herpes. A positive smear requires further evaluation.

Cultures for gonorrhea, herpes and group B streptococci – These cultures are recommended if there is history of previous positive cultures or if the patient is at risk.

Ultrasound has many uses in obstetrics. There are three levels of ultrasound in obstetrics – limited, basic and comprehensive. Guidelines for the use of limited ultrasound by nurses have been developed by the Association of Women’s Health, Obstetric and Neonatal Nurses (AWHONN). The limited exam is used for the following:

• Assessment of amniotic fluid volume
• Biophysical profile parameters
• Identification of fetal cardiac activity
• Location of placenta
• Confirmation of fetal presentation
• Certain procedures such as amniocentesis and external cephalic version

The basic examination is the most common ultrasound during pregnancy and includes:

• Fetal number
• Presentation
• Documentation of cardiac activity
• Placental location
• Amniotic fluid volume
• Assessment of gestation age – based on standardized fetal biometrics
• Anatomical survey of the fetus for malformations
• Evaluation of the maternal adnexa
• If abnormalities are suspected on previous examination or there is a suspected abnormality based on history or physical exam

Usually an abdominal or pelvic exam can determine fetal lie. In an obese patient or one who is difficult to examine it is sometimes impossible to determine whether the fetus is in a breech or vertex presentation or transfer lie. Ultrasound exam can readily determine fetal presentation. The American College of Obstetrics and Gynecologists has listed the following indications for ultrasound during pregnancy:

• Estimation of gestational age for patients with uncertain dates or verification of dates for those who will have elective termination or delivery of pregnancy
• Evaluation of fetal growth
• Vaginal bleeding in pregnancy
• Suspected ectopic pregnancy
• Identification of multiple gestation
• Diagnosis of molar pregnancy
• Detection of certain fetal abnormalities
• Detection of Hydramnios and oligohydramnios
• Diagnosis of fetal death
• Suspected abruption
• Confirmation of fetal lie, position, and presenting part
• Guidance for amniocentesis
• Biophysical evaluation
• Amniotic fluid index
• Observations of intrapartum events
• Estimation of fetal weight in preterm labor
• Follow-up of identified problems
• History of previous fetal anomaly
• Evaluation of late registrants for prenatal care

The BPP is a noninvasive, dynamic assessment of the fetus and fetal environment. The assessment is performed using real time ultrasound and the electronic fetal heart rate monitor. Parameters measured in this evaluation include:

• Fetal breathing movements
• Fetal movement
• Fetal tone
• Amniotic fluid index
• Nonstress test (NST)

FHR (fetal heart rate) reactivity, fetal movement, fetal breathing movements and fetal tone are acute biophysical markers and are believed to be initiated and regulated by complex, integrated mechanisms of the fetal central nervous system. Normal biophysical activity is indirect evidence that the portion of the CNS that controls that specific activity is intact. The absence of biophysical activities is difficult to interpret because it may reflect pathological depression or normal fetal periodicity. The measurement of amniotic fluid index is a marker of chronic fetal condition.

Investigators have shown an inverse relationship between non stress test (NST) and amniotic fluid index (AFI) findings. The lower the AFI the greater the incidence of non-reactive test, decelerations or perinatal morbidity or mortality.

Management guidelines are based on the BPP score and are as follows:

Amniotic fluid index is calculated by adding the largest vertical pockets of amniotic fluid in the four quadrants of the gravid uterus:

> 24 cm – increased is an indication for antepartum testing, including serial AFI measurements at least weekly. A complete ultrasound examination should be conducted to evaluate for associated fetal and placental anomalies, as well as for workup of polyhydramnios, including infection, diabetes and isoimmunizations.
10 to 24 cm – normal is a reassuring finding during fetal testing
5.1 to 9.9 cm – low normal should be evaluated taking into consideration the gestational age of the fetus. Because amniotic fluid volume peaks at 34 to 35 weeks gestation, an AFI of less than 10 cm should be reevaluated by additional measurements for the presence of associated conditions such as intrauterine growth retardation. A borderline value of 5.1 to 7.0 should be reevaluated every 3 to 4 days if all other findings remain normal
≤ 5 cm – decreased AFI values of 5 cm or less in a patient at term or post term indicate the need to deliver the fetus. When no amniotic fluid is found, a complete ultrasound examination should be conducted to rule out fetal anomalies. Rupture of the membranes maybe a cause for decreased or absent amniotic fluid.

The number of fetal movements decreases from early to late pregnancy in normal gestation. In pregnancies complicated by uteroplacental insufficiency, there is marked decrease in daily fetal movement count and a precipitous fall occurs in the period immediately preceding fetal death. Though the accuracy and reliability are variable.

The advantages of fetal movement counting are:

• Inexpensive
• Continuously available
• Relatively simple for patients to do

Parameters for normal daily fetal movement counts vary slightly from 3 to 10 in 1 hour to 3 in 30 minutes. Procedure varies with time of day, reference to mealtime, maternal position and hydration. The patient is instructed to be quiet, relaxed but awake, with an empty bladder, she places her hands on her abdomen and focuses on baby’s movements. It is sometimes helpful to use palpation to verify with the patient what sensations can be interpreted as fetal movement. Patients should continue to be aware of fetal movements and report the hourly observations if she perceives decreased fetal movement. It is important to note that fetal movement can occur without maternal recognition. A non-stress test is often performed if only one or two movements are felt within a 1-hour period. If the NST is reactive, further testing may not be done unless there are some other risk factors or if the patient again perceives a decrease in fetal movement. A non-reactive NST would be followed as soon as possible by a biophysical profile or contraction stress test.

UADV is a non-invasive technique used to assess the hemodynamics of vascular impedance. A high velocity umbilical artery diastolic flow is typically found in normally growing fetuses, whereas diminished flow is found in intrauterine growth retarded fetuses. The flow can be absent or even reversed in some cases of extreme intrauterine growth restriction. Abnormal flow velocity waveforms have been correlated with fetal hypoxia and acidosis and high perinatal morbidity and mortality. The flow indices generally measured are:

• Systolic to diastolic ratio

• Resistance index

• Pulsatility index

An abnormal flow is either an absent end diastolic flows or a flow index greater than 2 standard deviations above the mean for gestational age. Surveillance of suspected intrauterine growth restriction with umbilical artery Doppler velocimetry can achieve equivalent fetal and neonatal outcomes as primary antepartum surveillance based on the Non-Stress Test.

Direct visualization of the fetus can be done with an endoscope inserted into the amniotic cavity through the maternal abdomen. The procedure is done to directly view portions of the fetal anatomy in patients whose fetuses are at high risk for suspected abnormalities.

Fetoscopy has been used for directing skin biopsies and less frequently to obtain fetal blood in the diagnosis of fetal hemoglobinopathies. Fetoscopy may be used as an adjunct to laser ablation of connecting vessels in twin-twin transfusion syndrome. Real-time ultrasound is used during the procedure to guide the fetoscope to an appropriate area for viewing or blood sampling.

Fetoscopy is generally performed at 18 weeks gestation. Complications of this procedure are spontaneous abortion, preterm delivery, leakage of amniotic fluid, amnionitis and intrauterine fetal death.

 An analgesic may be given to the patient to limit fetal movements during the procedure. Post-procedure care includes monitoring of vital signs, administration of anti-Rh globulin as indicated, and teaching patients to report any pain, bleeding, amniotic fluid loss or fever.

            Amnioscopy is direct visualization of amniotic fluid through the fetal membranes with a cone-shaped hollow tube when the cervix is sufficiently dilated. It is done to identify meconium stained amniotic fluid. Amnioscopy is valuable in post date pregnancy when the possibility of post mature syndrome exists. The amnioscope may be used to visualize the presenting pat after rupture of membranes to obtain fetal blood sample for blood gas analysis.

Amniography is the injection of radiopaque agents into the amniotic sac to identify Hydramnios, oligohydramnios, placenta previa, soft tissue silhouette of the fetus and after a few hours of fetal swallowing – the fetal gastrointestinal tract. The fluid may become meconium stained after this procedure if the fetus is near term. Ultrasonography provides most of the same information without using ionizing radiation or injection of the amniotic sac.

MRI is a non-invasive tool that provides excellent visualization of soft tissue. MRI can be used to evaluate fetal structures, placental position and density, amniotic fluid quantity, maternal structures and metabolic or functional malformations.

The time required for the procedure ranges from 20 to 60 minutes, during which the patient must be very still. The procedure provides very specific information but has limited use because time involved and fetal movement often makes interpretation difficult.

Radiological assessment for fetal size and maturity and placental localization is seldom done since ultrasound diagnosis. A simple x-ray film of the abdomen and pelvis after 16 weeks gestation will most often identify fetal skeletal parts.

During the second half of pregnancy the number of fetuses can be seen in a multiple gestation. Anencephaly and hydrocephaly can be identified during the third trimester. Radiography is essentially done only in non-obstetrical applications such as for an intravenous pyelogram (IVP) or trauma.

Amniocentesis is the removal of fluid from the amniotic cavity by needle puncture. The procedure is performed under ultrasound guidance by insertion of a 20 to 22 gauge spiral type needle trans-abdominally to aspirate 5 to 20 ml of amniotic fluid. This is often performed to assess Lecithin/Sphingomyelin (L/S) ratio, fetal lung profile and amniotic fluid bilirubin. When genetic problems are suspected amniocentesis is performed as soon as possible at 16 to 20 weeks to allow for Karotyping and biochemical studies to be computed before the time limit for having an elective termination of pregnancy.

Amniocentesis later in pregnancy is most often performed to assess fetal well-being and maturity. In cases of isoimmunization the procedure maybe performed repeatedly to monitor the fetal condition. In high-risk pregnancies such as those with maternal diabetes, amniocentesis is done to assess fetal lung maturity, indicating the most opportune time for delivery. There is minimal risk of spontaneous abortion. Other risks include trauma to fetus or the placenta and bleeding into the maternal circulation.

Percutaneous umbilical blood sampling is achieved through the trans-abdominal insertion of a needle into a fetal umbilical vessel under ultrasound guidance. The ideal insertion point is near the placental insertion. 1 to 4 ml of blood is removed during the procedure and tested by the Kleinhauer-Betke procedure to ensure that the specimen is fetal blood.

The blood sample is used for determining Karotyping, direct Coombs, CBC, fetal blood type, blood gases, acid-base status for intrauterine growth restriction fetuses, detection of infection, and assessment and treatment of isoimmunization. Complications are unusual and are due to blood leakage from the puncture site, fetal bradycardia and chorioamnionitis.

Chorionic villus sampling (CVS) is the trans-cervical or trans-abdominal insertion of a needle into the fetal portion of the placenta to remove a small tissue specimen. The procedure is done between 10 and 12 weeks gestation under real-time ultrasound visualization. The aspiration cannula and obturator are passed through the cervix with care to avoid rupture of the amniotic sac.

This procedure can be done early in the first trimester to identify fetuses with genetic defects. Complications are rare but include vaginal spotting/bleeding, spontaneous abortion, rupture of membranes and chorioamnionitis. Rh-negative patients should receive Rh immune globulin because of the possibility of feto-maternal hemorrhage, which could result in isoimmunization.

The basis for the NST to assess fetal well-being is that the normal fetus will produce characteristic heart rate patterns. Average baseline variability and accelerations of FHR in response to fetal movement are reassuring signs. The FHR pattern is assessed by external monitoring without any stress or stimuli to the fetus. When hypoxia, acidosis or drugs depress the fetal central nervous system, there may be a reduction in baseline variability and absence of FHR acceleration with fetal movement.

The pattern can also happen when the fetus is asleep and it is sometimes necessary to monitor for an extra 20 to 30 minutes until the fetus is in a more active state. The efficacy of maternal ingestion of food or fluids to stimulate the fetus has not been established although it is the practice to feed the mother prior to the test. The advantage of the NST over the CST (contraction stress test) is that it can be performed in an outpatient setting.

The following are suggested guideline for interpretation of the NST:

• Reactive test - Two or more FHR accelerations above baseline of at least 15 bpm lasting at least 15 seconds in a 20-minute period; baseline rate is within the normal range and variability is average
• Non-reactive test - Absence of accelerations of FHR during the testing period
• Inconclusive test - Less than one acceleration above the baseline in a 20-minute period or one that is less than 15 bpm and lasts less than 15 seconds; variability less than 6 bpm or quality of FHR recording not adequate for interpretation

 The reactive test suggests that the fetus will be born in good condition if labor occurs in a few days. It should be repeated twice a week in high-risk patients. The non-reactive test should be followed as soon as possible by a contraction stress test. Patients with an inconclusive test may have the NST repeated in several hours or may have a Contraction Stress Test or biophysical profile.

The CST is a method of determining feto-placental respiratory reserve by observing the fetal heart rate response to uterine contractions. CST can be performed with endogenously produced oxytocin as stimulated by breast and nipple manipulation, or the test can be performed with an exogenous source of oxytocin administered by IV infusion. CST is contraindicated:

• Premature rupture of membranes
• Placenta previa
• Third-trimester bleeding
• Previous classical cesarean section
• Multiple gestation
• Incompetent cervix
• Hydramnios
• Risk for preterm delivery

The CST is highly reliable when it is negative. False negatives are rare. On the other hand, false positives can occur if Hyperstimulation patterns are unrecognized or if maternal position is supine resulting in hypotension and late decelerations. In contrast when there are no late decelerations in a patient in labor with previous positive CST, it may be indicative a correction of uteroplacental insufficiency in the interval between the test and labor and not a false positive. Interpretation of the CST for both nipple stimulation and IV oxytocin:

1. Negative test - Three uterine contractions in a 10-minute period without late decelerations; there is usually average baseline variability and accelerations of FHR with fetal movements
2. Positive test - Persistent late decelerations or late decelerations with more than half the contractions; may be associated with minimal or absent variability
3. Suspicious test - Late decelerations occurring with less than half the uterine contractions
4. Hyperstimulation - Contractions occurring more often than every 2 minutes or lasting longer than 90 seconds, or if there is apparent Hypertonus associated with contractions; if no late decelerations occur with the preceding, the test is interpreted as negative; if late deceleration is observed during or after excessive uterine activity, the test is not interpretable and is classified as Hyperstimulation because the stress is considered enough to exceed even normal uteroplacental reserve
5. Unsatisfactory - Quality of the recording is not sufficient to be sure that no late decelerations are present or where less than three uterine contractions have occurred in a 10-minute period; the test is not interpretable and cannot be used for clinical management

The purpose of Antepartum testing is used to verify fetal well-being. In the best of circumstances, the results of antepartum testing would be 100% accurate. However, each test has some level of error in prediction of fetal condition. Current testing measures accurately identify the healthy fetus but are not as sensitive in identifying truly compromised fetuses. A fetus who demonstrates a negative test result is very likely to be healthy, but the fetus who demonstrates a positive test result is not as likely to be truly compromised.

Because no test provides complete information regarding fetal well-being, several tests may be required. Strict protocols must be followed for the administration, interpretation and timely follow-up of the test results to avoid litigation.

Fetal Assessment is part of the process of providing prenatal care. Use of the fetal assessment methods discussed here have led to a decrease in perinatal mortality in pregnant women with identified risk-factors for uteroplacental insufficiency. However, half of unexpected fetal deaths occur in women with low-risk pregnancies without identifiable risk factors that would make them candidates for antepartum fetal testing.

Early identification of real or potential problems enables pregnant women to achieve the best possible obstetrical outcome. Patient education should be included in every aspect of the antepartum testing and interpretation of those results to improve compliance with therapy.

AWHONN Fetal Heart Monitoring Principles and Practice 2nd Edition, Kendall/Hunt Publishing Company, Dubuque (1993)

Douphinee, Joan Drukker, Antepartum Testing: A Challenge for Nursing, Journal of Perinatal & Neonatal Nursing 1987; 1(1): 29-48 Aspen Publishers, Inc.

Gegor, Carolyn L., and Paine, Lisa L., Antepartum Fetal Assessment Techniques: An Update for Today’s Perinatal Nurse, journal of Perinatal & Neonatal Nursing 1992; 5(4): 1-15 Aspen Publishers, Inc.

Murray, Michelle, Antepartal and Intrapartal Fetal Monitoring 4th Edition, Learning Resources International Inc. Albuquerque (1997)

Murray, Michelle, Urbanski, Patricia, Essentials of Fetal Monitoring, Learning Resources International Inc. Albuquerque (2000)

Queenan, John T., and Hobbins, John C., Protocols for High Risk Pregnancies 2nd Edition, Medical Economics Books, Oradell, New Jersey (1995)

Tucker, Susan, Mosby’s Fetal Monitoring and Assessment 4th Edition, St. Louis (2000)