Prenatal Diagnosis and Tests of Fetal Well-Being



Prenatal Diagnosis and Tests of Fetal Well-Being








FIRST AND SECOND TRIMESTER SCREENING

First trimester screening provides early testing for aneuploidy between 11 and 13 weeks. Screening includes biochemical markers for pregnancy-associated plasma protein A (PAPP-A), free β human chorionic gonadotropin (hCG), and ultrasound measurement of fetal nuchal translucency (FNT). The detection rate for first trimester screening is 80%, with false-positive rate of 5%. First trimester combined screening can also screen for trisomy 18, and NT alone may detect trisomy 18, trisomy 13, and other chromosomal abnormalities. The maternal triple or quadruple screen tests are offered to pregnant women during their second trimester to identify risks for chromosome disorders such as Down syndrome (trisomy 21); major birth defects, including open neural tube defects such as spina bifida; placental insufficiency; and oligohydramnios. The evaluation consists of three or four separate blood protein tests done on maternal serum between 15 and 21 weeks of gestation: MS-AFP is decreased in Down syndrome and neural tube defects, unconjugated estriol (E3) is decreased in Down syndrome, and β-hCG is increased in Down syndrome. Results are reported as multiples of the median (MoM). The triple screen detection rate for Down syndrome is approximately 69% (false-positive rate of 5%), 80% for neural tube defects, and 60% for trisomy 18 (false-positive rate of 0.2%). The detection rate for Down syndrome with quadruple screen is 81%. Combining first and second trimester screening improves the detection rate to 94% to 96%, with a 5% false-positive rate. Quad screen includes AFP, hCG, E3, and inhibin A.

The maternal triple and quadruple tests are screening tests; therefore, an abnormal (positive) result is not diagnostic, and further testing with ultrasound, amniocentesis, and genetic counseling is indicated. The markers can be positive in normal variations such as multiple births or miscalculated gestational age.

Ultrasound testing is a method of assessing fetal well-being that has become a diagnostic tool for assessment of fetal age, health, growth, and identification of anomalies. Level I ultrasound assesses gestational age, number of fetuses, fetal death, and the condition of the placenta. Level II ultrasound assesses specific congenital anomalies or abnormalities. In some diagnostic centers, fetal echocardiography is also available. An additional ultrasound marker for Down syndrome is the absence or hypoplasia of nasal bones in fetuses at 11 to 14 weeks’ gestation. When combining this marker





with PAPP-A, β-hCG, and FNT, the detection rate for Down syndrome is significantly increased. Color-enhanced Doppler ultrasound is used to measure the velocity and direction of blood flow in fetal and uterine anatomy, to provide information about placental function, and as an especially good predictor of outcome for fetuses that are small for gestational age (see Chapter 13). American Congress of Obstetricians and Gynecologists (ACOG) guidelines support the use of umbilical artery Doppler surveillance in the management of intrauterine growth restriction. Middle cerebral artery Doppler assessment is used to predict fetal anemia in at-risk pregnancies. Ultrasound also includes three-dimensional (3D) and four-dimensional (4D) technologies. In fetal medicine, the use of these technologies may provide assessment of fetal anomalies of the limbs, thorax, spine, central nervous system, and face. Some centers use 4D ultrasound for guided needle procedures such as amniocentesis and cordocentesis to improve accuracy of the procedure. Three- and four-dimensional ultrasound is also used to evaluate fetal heart function and structure.








TABLE 15.1 Maternal Fetal Testing During Pregnancy

































































































































































































Tests and Procedures


Indications


Follow-Up and Interventions for Positive Test Results


First Trimester (1-14 wk)


Human chorionic gonadotropin (hCG) levels


Confirm normal progress of pregnancy or if suspected for ectopic pregnancy, threatened or missed abortion, or to monitor success of insemination or in vitro fertilization


Levels increase 66% or double every 36-48 hr; if levels off or decline, may be ectopic or miscarriage. Usually, two tests are done 48 hr apart and often follow-up with ultrasound. Dramatic increase may indicate multiple fetuses.


Prenatal Profile


Complete blood count (CBC) or hemoglobin/hematocrit (Hb/Hct)


Detection of anemia defined by the Centers for Disease Control and Prevention as below 11/33 in first trimester. CBC with red blood cell (RBC) indices for detection of carrier state of hemoglobinopathies


Anemia is treated with iron supplementation.


Follow-up test is repeated at 28 wk and as indicated.


Hb electrophoresis, Sickledex, or high-powered liquid chromatography and isoelectric focusing for those at risk for hemoglobinopathies: African Americans (sickle cell disease); Italian, Greek, or Corsican (β-thalassemia); and Southeast Asian (α-thalassemia)


ABO, Rh


Blood typing to identify Rh-negative women and ABO group for incompatibility


Intervention for Rh-negative women is to receive Rh immunoglobulin (RhoGAM) prophylaxis. Identification of type O women for consideration of ABO incompatibility in neonatal jaundice


Rubella immunity status


Identify nonimmune women to avoid possible exposure to rubella and report any possible exposure; considered immune if there is a documented dose of rubella vaccine or an immune serologic test result


Instruction to nonimmune women to report exposure to rubella for immunoglobulin prophylaxis; immunization is recommended postpartum.


HIV


All should be counseled for screening to reduce perinatal transmission to infant.


Positive results indicate need to recommend antiretroviral treatment and discussion about possible cesarean delivery to reduce perinatal transmission as well as avoidance of breast-feeding if indicated.


Follow-up testing may be needed for high-risk individuals.


Syphilis antibody immunoglobulin G (IgG)


Identify syphilis infection in pregnant women to reduce fetal infection


Positive result indicates need for treatment with penicillin or other antibiotic if allergy exists.


Venereal Disease


Research Laboratory



Follow-up testing may be needed for high-risk individuals.


Hepatitis B surface antigen


All pregnant women should be screened to identify chronic disease carriers.


Follow-up includes immunization of those with negative test but considered high risk for acquiring hepatitis B during pregnancy, and positive results indicate need to immunize infant with hepatitis B immune globulin and hepatitis B vaccine as soon as possible after birth.


Antibody screen


All pregnant women should be screened to identify isoimmunized women.


If positive result, test is repeated to identify specific maternal antibody (e.g., anti-D, C, c, E, e, Kell, Duffy, Kidd).


Varicella status


All pregnant women unless reliable history of varicella as a measure of immunity


If nonimmune and with history of exposure, needs varicella titer within 24-48 hr; if needed, varicellazoster immune globulin given within 96-144 hr of exposure. Varicella vaccine can be given postpartum.


Papanicolaou test


All pregnant women without a documented normal Pap test within the past 6 mo


Positive results may need follow-up with a colposcopy or repeat Pap test. Most often, treatment is delayed until postpartum.


Urinalysis, urine culture


All pregnant women are screened for asymptomatic bacteriuria.


Positive results of > 100,000 colonies of single organism should be treated and high-risk patients screened each trimester.


For At-Risk Patients


Wet prep


All pregnant women at risk for preterm birth


Positive clue cells, trichomonads, or Candida indicates need for treatment.


Gonorrhea


Pregnant women with risk factors


Positive results should be treated with appropriate antibiotic therapy, and test of cure should be obtained.


Chlamydia


Pregnant women with risk factors


Positive results should be treated with appropriate antibiotic therapy, and test of cure should be obtained.


Genital herpes culture if active lesion


All pregnant women with active lesion


Positive test may be treated with antiviral medication, and patient may be counseled regarding risks and benefits of cesarean delivery.


Tuberculosis test


High-risk pregnant women or symptomatic


Follow-up may include chest x-ray with shielding preferred after 12 wk and treatment with medications during pregnancy.


Chorionic villus sampling (CVS)


Pregnant women at risk for fetal genetic or biochemical disorders or those with abnormal ultrasound


Positive test requires follow-up genetic counseling or discussion of treatment options.


Fetal nuchal translucency (FNT; may be combined with pregnancy-associated plasma protein A [PAPP-A], β-hCG to increase detection rate)


Any pregnant woman presenting by 11-13 wk can be screened, particularly desired screening for Down syndrome, trisomy 13, trisomy 18, Turner’s syndrome.


Positive test follow-up with counseling regarding CVS or amniocentesis for definitive diagnosis. Positive test can be associated with other fetal conditions if no chromosomal abnormality.


Fetal cell-free nucleic acids—screening test to detect trisomy 13, 18, or 21


At-risk women include age 35 yr or older, ultrasound findings show increased risk for aneuploidy, history of a child with trisomy, or positive first or second trimester screening. Can be offered after 10 wk of gestation


Positive test indicates need for genetic counseling and further testing for confirmation of test results.


Carrier testing for cystic fibrosis (CF). American Congress of Obstetricians and Gynecologists (ACOG) recommends standard screening test should include 25 disease-causing mutations for CF


ACOG recommends that all pregnant women of northern European and Ashkenazi Jewish origin be offered carrier testing for CF as standard of care. Also, individuals with family history of CF, reproductive partners of individuals with CF, couples with one or both Caucasian partners who are pregnant or planning a pregnancy


Genetic counseling for positive test


Those with a negative screening should be aware that they may be a carrier for mutation not included in the test.


Ultrasound


Pregnancy confirmation Viability, rule out (R/O) ectopic pregnancy, gestational age, fetal assessment


Follow-up ultrasound level I or II


PAPP-A


Screening for chromosomal abnormalities (can be used in combined testing with FNT and β-hCG based on maternal age)


Lower in pregnancies if fetus has Down syndrome


Preimplantation genetic diagnosis


Genetic testing of an early embryo at 6- to 8-cell stage (3 d after fertilization) examined for aneuploidy, structural chromosomal abnormalities, single-gene disorders, X-linked disorders


Embryos are implanted after genetic testing rules out abnormalities.


Gestational glucose screening—1 hr


Screening for gestational diabetes risk


Positive tests require a 3-hr oral (100 g) glucose tolerance test for diagnosis of gestational diabetes.


Second Trimester (15-28 wk)


Triple screen (hCG, unconjugated estriol [uE3], maternal serum alpha-fetoprotein [AFP]) Quadruple screen adds inhibin A


Screening for Down syndrome, trisomy 18, and possibly Turner’s syndrome, triploidy, Smith-Lemli-Opitz syndrome. Low inhibin A increases the detection of Down syndrome and trisomy 18.


Genetic counseling, evaluation by perinatologist, possibly level II ultrasound and amniocentesis


Amniocentesis


Amniotic fluid studies of fetal genetics to identify abnormalities, karyotyping to identify chromosomal disorders


Genetic counseling


Ultrasound 18-20 wk, level I or II


Facilitate amniocentesis, determine or confirm estimated date of delivery and fetal viability, R/O abnormal pregnancy, intrauterine growth retardation (IUGR), congenital anomalies, oligo- or polyhydramnios. Identify placental location, cervical length, multiple gestation, amniotic fluid index (AFI) Level II: assess specific anomalies in fetal anatomy such as congenital heart defects, omphalocele, anencephaly; identify ultrasound markers that increase risk for genetic abnormalities


Positive results may require repeat or serial ultrasound evaluations, MRI, 3D or 4D ultrasound, or genetic counseling.


Umbilical artery Doppler tests


Identify abnormal placental function in at-risk pregnancies such as pregnancy-induced hypertension, IUGR. Identify fetal acidosis, hypoxia.


Positive results may indicate need for further monitoring or need to deliver infant.


Fetoscopy


Identify fetal developmental defects, blood disorders such as hemophilia A and B, sickle cell anemia; perform therapeutic interventions, sample fetal tissue


Positive results may require interventions or need for care conference for plan of delivery of abnormal infant.


Percutaneous umbilical blood sampling


Need for fetal blood sampling with less risk than fetoscopy. Identify such disorders as hemophilia, hemoglobinopathies, infections, drug levels, chromosomal abnormalities, cord blood pH.


Positive results may indicate need for treatment, immediate delivery, or genetic counseling.


Serum-Integrated Screening


PAPP-A from first trimester screening Alpha fetoprotein monoclonal antibody (AFP3), inhibin A in second trimester


Prenatal screening for Down syndrome


Genetic counseling, evaluation by perinatologist, possibly level II ultrasound and amniocentesis


Fully Integrated Screening


FNT and PAPP-A in first trimester


AFP3, inhibin A in second trimester


Prenatal screening for Down syndrome


Genetic counseling, evaluation by perinatologist, possibly level II ultrasound and amniocentesis


Third Trimester (29-40 wk)


Fetal fibronectin


With symptoms of preterm labor, if intact membranes and <3 cm dilation, may help to predict preterm delivery


Positive results predict probable delivery in next 7-14 d.


Nonstress test, contraction stress test, oxytocin challenge test, breast stimulation test, fetal activity acceleration determination


Assess fetal heart rate (FHR) in response to fetal movement or contractions to assess fetal well-being, fetal hypoxia, tolerance to labor


Positive result may indicate need for further testing, induction of labor, or immediate delivery.


Biophysical profile


Used in high-risk pregnancy to assess fetal well-being or diagnose fetal hypoxia or distress


Positive result may indicate need for further testing, induction of labor, or immediate delivery.


Amniocentesis


Determine fetal lung maturity, fetal infections or fetal hematologic disorders, previous history of erythroblastosis


Lecithin/sphingomyelin ratio of >2 indicates lung maturity and, if delivered, lessens chance of respiratory distress syndrome


Group B streptococcus screening


All pregnant women should be screened for anogenital group B streptococcus colonization between 35 and 37 wk.


Positive results indicate colonization and indication for antibiotic prophylaxis intrapartum.


Fetal oxygen saturation (FSpO2) monitoring


Indicated if FHR monitoring is not reassuring or difficult to interpret; can be used if membranes are ruptured, vertex presentation, >36 wk


FSpO2 <30% for >10 min is probably hypoxemia, indicating need for intervention or delivery.


Ultrasound


Indicated to determine fetal position, placenta previa, abruption or maturity, FHR if unable to Doppler fetal heart tones, assess fetal growth, AFI, estimate fetal weight, R/O multiple gestation, anomalies


To determine fetal well-being, need for immediate delivery or induction of labor, or need for follow-up ultrasounds


Amniotic fluid fern test AmniSure detects human protein in amniotic fluid (PAMG-1)


Determine rupture of membranes


Positive results may indicate need to deliver within 24 hr or induction of labor if no spontaneous labor.


Human placental lactogen


High-risk pregnancies to evaluate placental function


Decreased or falling levels may indicate need for further testing of fetal well-being.


Electronic fetal monitoring


Indicated antepartum to evaluate fetal well-being for high-risk pregnancies, during or after procedures, for symptoms of preterm labor, decreased fetal movement, maternal drug administration. Indicated intrapartum intermittently for low-risk pregnancies or continuously for high-risk pregnancies, during oxytocin administration, epidural anesthesia, or other interventions


Signs of fetal distress warrant interventions to improve fetal oxygenation or immediate delivery.


Although magnetic resonance imaging (MRI) is used at some prenatal centers, it is still under investigation for diagnostic evaluation in pregnancy, especially in the final trimester (see Chapter 16). MRI is most often used to define central nervous system defects. Some of the advantages of MRI during pregnancy are that it is a noninvasive technique, it permits easy differentiation between fat and soft tissue, it does not require a full bladder, and it can show the entire fetus in one scan. Currently, MRI confirms fetal abnormalities found by ultrasound and can be used for pelvimetry, placental localization, and determination of size. Fetal MRI is used at medical centers that specialize in fetal diagnosis and treatment (particularly those that perform fetal surgery). Ultrafast MRI is used for evaluation of congenital anomalies that are potentially correctable, such as congenital diaphragmatic hernia, neck masses that result in airway obstruction, myelomeningocele, and cleft lip and cleft palate. MRI is especially useful for definition of maternal anatomy in cases of suspected intra-abdominal or retroperitoneal disease.



• Maternal Serum Alpha-Fetoprotein (MS-AFP)

The measurement of MS-AFP is offered between 15 and 21 weeks’ gestation when AFP concentration rises dramatically, and the test is used to screen for fetal abnormalities and neural tube defects. Approximately 90% of infants with neural tube defects are born to parents who have no recognized risk factors for the disorder. Maternal quadruple screen (MoM: AFP, hCG, and E3 and inhibin A) aids in identifying Down syndrome risk. The test also detects complications of pregnancy, and AFP serves as a tumor marker in nonpregnant women. This test is also used to detect pregnancy complications such as intrauterine growth retardation, fetal distress, and fetal demise. It can also be used to diagnose and monitor hepatocellular, testicular, ovarian, and malignant liver diseases.


Reference Values


Normal

At 15 to 21 weeks’ gestation: 10 to 150 ng/mL or 10 to 150 µg/L

Normal adult level: <10 ng/mL or <10 µg/L

MoM: 0.5 to 2.5 (calculated by dividing the patient’s AFP by median AFP for a normal pregnancy at the same gestational age). An AFP MoM <2.5 is reported as screen negative. A screen negative result indicates that the calculated AFP MoM falls below the cutoff of 2.5 MoM. A negative screen does not guarantee absence of a neural tube defect.




Interfering Factors



  • Obesity causes low MS-AFP.


  • Race is a factor: MS-AFP levels are 10% to 15% higher in African Americans and are lower in Asians than in Caucasians.


  • Insulin-dependent diabetes results in low MS-AFP levels.




• Circulating Cell-Free DNA

Fetal “cell-free” nucleic acids (DNA and RNA) are found in the maternal circulation, are unique to the current pregnancy, and are thought to be from the placenta. Circulating cell-free DNA is taken from the plasma of maternal whole blood. Testing is available to detect fetal aneuploidies for chromosome 21, 18, and 13. The test can also determine X and Y chromosomes. This noninvasive test can be performed as early as 10 weeks’ gestation with results available in 1 week. ACOG recommends that women, regardless of age, be offered prenatal screening for aneuploidy by screening or invasive testing. Cell-free DNA testing is one method of noninvasive screening for women at risk for aneuploidy. It is not recommended as a screening test for low-risk women at this time due to lack of testing in the low-risk population.

Indications for cell-free DNA testing for aneuploidy:



  • Maternal age of 35 years or older at delivery


  • Ultrasound findings of fetus indicating increased risk for aneuploidy



  • Prior history of pregnancy with trisomy


  • Positive first or second trimester sequential or integrated screen or quad screen


  • Parental balanced Robertsonian translocation with increased risk of fetal trisomy 13 or trisomy 21


  • Cell-free DNA testing can identify 99% of Down syndrome, 99.9% of trisomy 18, and 91.7% of trisomy 13.


Reference Values


Normal

Negative test indicates no evidence of aneuploidies for 21, 18, or 13.


Positive

Trisomy 21, trisomy 18, or trisomy 13 detected





• Hormone Testing

Normally, the amounts of all steroid hormones increase as pregnancy progresses. The maternal unit responds to altered hormone levels even before the growing uterus is apparent. Serial testing may be done to monitor rising levels of a particular hormone over a period of time. Decreasing levels indicate that the maternal-placental-fetal unit is not functioning normally. Biochemical analyses of several hormones can be used to monitor changes in the status of the maternal-fetal unit (see Chapters 3 and 6).



  • In early pregnancy, hCG in maternal blood provides evidence of a viable pregnancy. The hCG in maternal serum is measured as a sensitive pregnancy test (the hCG level increases 66% to 100% every 48 hours during pregnancy). Also, it is used to monitor the success of in vitro fertilization or insemination, to diagnose trophoblastic tumor, to diagnose ectopic pregnancy (indicated by a
    decrease in hCG over a 48-hour period), and to screen for Down syndrome in pregnancy. For further discussion of pregnancy tests, see Chapter 6.


  • Together with prolactin and luteinizing hormone (LH), hCG prolongs the life of the corpus luteum once the ovum is fertilized. It stimulates the ovary for the first 6 to 8 weeks of pregnancy, before placental synthesis of progesterone begins. Its function later in pregnancy (in maternal blood) is unknown.


  • PAPP-A, a circulating placental protein, has been shown to increase the stimulatory effects of placental insulin-like growth factors. Decreased serum levels in the maternal circulation in the first 10 weeks after conception are associated with uncomplicated full-term low birth weights. PAPP-A levels are detectable within 30 days after conception and slowly increase throughout the first 30 weeks of gestation. Maternal serum levels are 0.43 µg/L (12 pmol/L). Increased PAPP-A occurs in Down pregnancy.


  • Late in pregnancy, the levels of E3 and human placental lactogen (hPL) in maternal blood reflect fetal homeostasis. hPL is a protein hormone produced by the placenta. Testing of hPL evaluates only placental functioning. Blood testing of the patient usually begins after the 30th week and may be done weekly thereafter. A concentration of 1 µg/mL (46 nmol/L) hPL may be detected at 6 to 8 weeks of gestation. The level slowly increases throughout pregnancy and reaches 7 µg/mL (324 nmol/L) at term before abruptly dropping to zero after delivery. hPL functions primarily as a fail-safe mechanism to ensure nutrient supply to the fetus, for example, at times of maternal starvation. However, it does not appear to be required for a successful pregnancy outcome (see Chapter 6).


• Estriol (E3)

E3 is the predominant estrogen in the blood and urine of pregnant women and is of fetal origin. Normal production serves as a measure of the integrity of the maternal-fetal unit and of fetal well-being.

This test is used during pregnancy to evaluate fetal disorders and is part of the maternal triple screen. Declining serial values indicate fetal distress, although in some high-risk pregnancies, E3 is not reduced. A single determination cannot be interpreted in a meaningful fashion. E3 is decreased in Down syndrome and in trisomy 18. An elevated serum or unconjugated E3 above 3 multiples of the gestational age mean, or with an absolute value of >2.1 ng/mL, can indicate pending labor or fetal congenital adrenal hyperplasia.


Reference Values


Normal






































Weeks of Gestation


E3 (ng/mL)


SI Units (nmol/L)


28-30


38-140


132-485


32


35-330


121-1144


34


45-260


156-901


36


46-350


159-1277


38


59-570


214-1976


40


90-460


306-1595


Levels peak in the middle or late afternoon. The day-to-day variation is 12% to 15%.






Interfering Factors

Administration of radioactive isotopes within the previous 48 hours interferes with this test.

Estrogen or progesterone therapy can interfere with test results.

Drugs that can cause decreased levels include vitamins and some phenothiazines.

Tetracycline can increase levels.



• Human Placental Lactogen (hPL) (Chorionic Somatomammotropin)

hPL is a growth-promoting hormone of placental origin and is similar to hCG (see Hormone Testing).

This test is used to evaluate placental function as an index of fetal well-being in at-risk pregnancies. Low hPL levels are associated with intrauterine growth retardation. Falling levels indicate a poor prognosis. The level of hPL correlates best with placental weight, but the clinical significance of this hormone is controversial.


Reference Values


Normal

Normal maternal serum: <0.5 µg/mL (<0.5 mg/L or <25 nmol/L)

1st Trimester: 0.2-2.1 µg/mL

2nd Trimester: 0.5-6.7 µg/mL

3rd Trimester: 4.5-12.8 µg/mL

Men and nonpregnant women: 0-0.1 µg/mL





Interfering Factors

Administration of radiopharmaceuticals 24 hours before venipuncture interferes with this test.



• Fetal Fibronectin (fFN)

Fetal fibronectin (fFN) is abundant in amniotic fluid and may be useful in the diagnosis of ruptured membranes. The detection of fFN in vaginal secretions before membrane rupture may be a marker for impending preterm labor within the next 7 to 14 days.

This test helps to predict a preterm delivery when the presenting symptoms are questionable so that early intervention (e.g., tocolytics, corticosteroids, transport to a tertiary center) can be initiated when indicated. This test is for women with intact membranes and cervical dilation <3 cm. fFN is secreted in early pregnancy to help attach the fertilized egg to the implantation site in the uterus, but it is not secreted after 22 weeks until near term. This test detects preterm labor from 24 until 34 weeks’ gestation.



Reference Values


Normal

Negative: <0.05 µg/mL or <0.05 mg/L (delivery is unlikely to occur within 14 days)

Positive: >0.05 µg/mL or >0.05 mg/L (delivery within 7 to 14 days)




Interfering Factors



  • Vaginal bleeding


  • Ruptured membranes


  • Sexual intercourse within 24 hours of collection



TESTS TO PREDICT FETAL OUTCOME AND RISK FOR INTRAUTERINE ASPHYXIA


• Contraction Stress Test (CST)

This test is done in a hospital or clinic setting to assess fetal heart rate (FHR) in response to uterine contractions through electronic fetal monitoring.


Reference Values


Normal

The test result is negative if there are no late decelerations associated with at least three contractions within a 10-minute period.

A normal (negative) contraction stress test (CST) implies that placental support is adequate, that the fetus is probably able to tolerate the stress of labor should it begin within 1 week, and that there is a low risk for intrauterine death due to hypoxia.





• Oxytocin Challenge Test (OCT); Nipple Stimulation Test; Breast Stimulation Test (BST)

These tests are performed after 28 weeks of gestation, when a nonstress test (NST) is nonreactive or a CST is either positive or unsatisfactory. Continuous external fetal monitoring is used. Because uterine contractions are associated with a reduction in uteroplacental blood flow, spontaneous, oxytocin-induced, or nipple stimulation-induced contractions with a frequency of three in 10 minutes may be used clinically as a standard test of fetoplacental respiratory function. Stress of this magnitude has been proved clinically useful in separating fetuses with suboptimal oxygen reserve from those with adequate reserve (the vast majority), and it does not significantly compromise the normal fetus.


Reference Values


Normal

The test result is negative if there are no late decelerations associated with at least three contractions within a 10-minute period.

A normal (negative) result is reassuring; it implies that placental reserve is sufficient should labor begin within 1 week. There is a false-normal rate of 1 to 2 per 1000 pregnancies. The procedure is usually repeated weekly.






• Nonstress Test (NST)

The NST can be performed in a hospital, a clinic, or possibly a home care setting. The NST is a screening test and can be safely done once a week. Test results reflect the functions of the fetal brainstem, autonomic nervous system, and heart.


Reference Values


Normal

Negative result: reactive NST

ACOG criteria for a reactive NST (with or without stimulation): two or more accelerations of FHR, peaking at least 15 beats/minute above the baseline FHR and lasting at least 15 seconds from baseline to baseline, within a 20-minute period

Sep 25, 2018 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Prenatal Diagnosis and Tests of Fetal Well-Being
Premium Wordpress Themes by UFO Themes
%d bloggers like this: