Chapter 15

Indications for fetal diagnostic procedures: concurrent maternal factors

Prepregnancy body mass index (BMI) less than 18.5 kg/m2 Prepregnancy BMI 25 kg/m2 or higher Inadequate weight gain or poor pattern of weight gain Excessive weight gain Use of drugs (legal, including prescribed, over-the-counter, and herbal; illegal), alcohol, tobacco

Indications for Chronic Villus Sampling

CVS is usually performed between 10 and 13 weeks of gestation to diagnose fetal chromosomal, metabolic, or DNA abnormalities.

ultrasound and first trimester

Transvaginal ultrasound is often used during the first trimester because the uterus, gestational sac, embryo, ovaries, and fallopian tubes are deep in the pelvis.

advantages of ultrasound

clear visualization of fetus and surround structures, noninvasive, results are immediate, portable

Low Levels of AFP

Chromosomal trisomies (e.g., Down syndrome) Gestational trophoblastic disease Normal fetuses in conjunction with the following: Overestimation of gestational age Incorrect maternal weight (higher than true weight)

Indications for fetal diagnostic procedures: obstetrics factors

History of low-birth-weight (<2500 g [5 lb, 8 oz]) or preterm (<37 completed weeks of pregnancy) infant Multifetal pregnancy Malpresentation (breech, shoulder) Previous fetal loss or birth of infant with congenital anomaly Previous infant ≥4000 (8 lb, 13 oz) g at birth Hydramnios (≥2000 mL at term; amniotic fluid index ≥24-25 cm [Cunningham et al., 2010]) Oligohydramnios (<500 mL at term; amniotic fluid index <5) Decrease in or absence of fetal movements Uncertainty about gestational age Suspected intrauterine growth restriction Discordant (unequal) fetal growth of twins Postmaturity (>42 weeks) Preterm labor (>20 weeks but <37 completed weeks of gestation) Grand multiparity (>5 pregnancies)

accurate gestational age

Knowing the true gestational age is needed when screening for the level of maternal serum alpha-fetoprotein (MSAFP), which changes with fetal age. Accurate gestational age is also important if intrauterine growth restriction is suspected or the expected date of delivery is uncertain.

Purpose of AFP screening

Low levels of MSAFP are associated with chromosomal anomalies, such as trisomy 21 (Down syndrome). The most common cause of elevated AFP is failure of the embryonic neural tube or anterior body wall to close properly. The most common open neural tube defects are anencephaly, in which the cranial vault is absent and most of the brain is undeveloped, and spina bifida, which has a wide range of severity

Elevated Levels of Alpha-Fetoprotein (AFP)

Open neural tube defects (anencephaly, spina bifida) Esophageal obstruction Abdominal wall defects (omphalocele, gastroschisis) Increased amount leaked by fetal kidney (hydronephrosis) Threatened abortion Fetal demise Normal fetus in conjunction with one or more of the following: Amniotic fluid contaminated with fetal blood during amniocentesis Underestimation of fetal age Multifetal gestation Incorrect maternal weight (lower than true weight) Maternal insulin-dependent diabetes

indications for fetal diagnostic procedures: medical conditions

Preexisting diabetes mellitus or gestational diabetes Hypertension (chronic or preeclampsia) Acute or nonacute infections (e.g., pyelonephritis) Sexually transmitted diseases Severe anemia Parents carry or express a genetic disorder (e.g., sickle cell anemia, cystic fibrosis)

Body measurement and gestational age

Several body measurements are done to estimate gestational age during the last half of pregnancy, such as biparietal diameter, femur length, and abdominal circumference. Sequential assessments of multiple fetal measurements will help date the pregnancy more accurately than a single measurement. Estimating fetal age by ultrasound after 32 weeks is subject to major error. At this time the fetus may be evaluated for other signs of maturity and for signs of excessive or reduced growth rate

Second and third trimesters

Transabdominal ultrasound is common during the second and third trimesters because the uterus is out of the pelvis and accessible. Transvaginal ultrasound continues to be useful to evaluate the cervical and lower uterine areas.

Doppler ultrasound blood flow assessment

When an ultrasound wave is directed at an acute angle to a moving target, as with blood flowing through a vessel, the frequency of echoes changes as the cardiac cycle goes through systole and diastole. This change, referred to as the Doppler shift, indicates forward movement of blood within a vessel and resembles gentle hills and valleys that remain above the baseline.

comprehensive ultrasound

in the second trimester is used to evaluate the fetus when risk factors are present or the basic examination shows abnormal findings. Examples include prior birth of an infant with anomalies or abnormal clinical findings such as hydramnios (excessive amniotic fluid), oligohydramnios (insufficient amniotic fluid), or abnormal levels of MSAFP. Fetal anatomy is systematically examined to identify major system and organ structures. Anomalies that can be detected with comprehensive ultrasound include most open neural tube defects such as myelomeningocele and anencephaly (nonclosure of spinal cord); abdominal wall defects such as gastroschisis and omphalocele; malformed kidneys; hydrocephalus; obstruction in fetal bowel and urinary systems; cleft lip and palate; and limb abnormalities. Maternal obesity may limit accuracy of ultrasound in pregnancy

Alpha-fetoprotein (AFP)

is the main protein in fetal plasma

Color Doppler

is useful to clarify the relationships of body structures to each other. Nondirectional color Doppler imaging uses a single color to identify structures such as the number of vessels in the umbilical cord. Directional color Doppler uses two or more colors to determine the direction and speed of blood flow and pulsations within cardiovascular structures It helps determine whether the heart has normal structure and whether major vessels have correct relationships to the heart chambers. Color Doppler imaging can determine blood flow and pulsations within umbilical cord vessels and other major vessels such as cranial vessels.

Purpose third trimester amniocentesis

may be used to determine fetal lung maturity or to evaluate fetal hemolytic disease that is often caused by Rh incompatibility. Reduction amniocentesis is a variation in which excess amniotic fluid is removed and discarded when hydramnios occurs. Samples of the fluid removed in a reduction amniocentesis may be evaluated for presence of infection or for substances that help evaluate fetal condition.

Ultrasound

procedures in obstetrics use real-time scanning in which a rapid sequence of fixed images is displayed on the screen, showing movement in body tissues as it happens. This technique allows the observer to detect movement such as fetal heartbeat, fetal breathing activity, and fetal body movement. Still images are captured for purposes such as gestational age calculation using multiple measures (Figure 15-1). Both video images may be captured for medical records as well as memories for the parents. Three-dimensional images may be captured for greater detail of the fetal body (Figure 15-2).

Advantages of Chronic Villus Sampling

the fetal cell in the villi are rapidly dividing and usually available earlier than amniocentesis

If maternal serum samples are positive...

the woman should be offered additional testing, such as amniocentesis (withdrawal of amniotic fluid through the abdomen) for karyotyping or additional ultrasound to look for physical characteristics associated with the chromosome defects.

Procedure for Chronic Villus Sampling

the woman should receive both counseling about the procedure itself and genetic counseling about the specific defect for which CVS is being performed. The benefits and limitations of the procedure should be carefully explained, and a signed informed consent is obtained. CVS can be performed by a transcervical or transabdominal approach (AAP & ACOG, 2007; Cunningham et al., 2010; Wapner et al., 2009). In the transcervical technique, a flexible catheter is inserted through the cervix and a sample of chorionic villi is aspirated (Figure 15-5). In the transabdominal technique, a needle is inserted through the abdominal and uterine walls to collect chorionic tissue. After the procedure, the woman is shown the fetal heart motion, and maternal vital signs are assessed. Heavy bleeding or the passage of amniotic fluid, clots, or tissue suggests possible miscarriage and should be reported. The woman should rest at home for several hours after the procedure.

disadvantages of ultrasound

they can not detect every possible anomaly, may be cost issue if patient does not have insurance

Indications for prenatal tests

to detect congenital anomalies, to evaluate the condition of the fetus if the pregnancy is high risk and allow appropriate intervention, and to provide baseline information such as a more accurate gestational age

Purpose of Chronic Villus Sampling

uses transcervical or transabdominal sampling to obtain villi to diagnose fetal chromosome or metabolic abnormalities. It cannot be used to diagnose anomalies for which amniotic fluid is essential, such as open neural tube or body wall defects, which require measuring AFP levels

purpose of ultrasound during the second and third trimester

•Confirm fetal viability •Evaluate fetal anatomy •Estimate gestational age •Assess progress of fetal growth over a series of scans •Compare growth of fetuses in multifetal gestations •Evaluate amniotic fluid volume (see also "Biophysical Profile," p. 311) •Determine location and relation of the placenta and umbilical cord to each other and the insertion of the cord into the fetal abdomen •Determine fetal presentation •Guide needle placement for procedures such as amniocentesis or percutaneous umbilical blood sampling

Limitations of MSAFP

•It is a screening test and must be viewed as the first step in a series of diagnostic procedures that are indicated if abnormal concentrations are found. Parents must decide about whether to proceed each time another diagnostic test is offered. •Benign conditions, such as inaccurate estimation of gestational age, can result in apparently abnormal levels, causing the parents greater anxiety and expense if follow-up tests are indicated. •Timing imposes limits. Evaluation is best performed between 16 and 18 weeks of pregnancy, but many women do not seek prenatal care until well after the 18th week, thus limiting their options. •Because closed defects that are covered by skin do not produce elevated levels of AFP, normal levels of AFP do not guarantee that the baby will be free of structural anomalies.

Indications for fetal diagnostic procedures: demographics

Maternal age <16 or >35 years Poverty Nonwhite (greater risk for prematurity or neonatal or infant death) Inadequate prenatal care (initial visit after 20 weeks of gestation or fewer than five prenatal visits to physician or nurse-midwife)

Transabominal ultrasound

The mother is positioned on her back with her head and knees supported by pillows. Her head should be elevated, and she should be turned slightly to one side with a wedge or rolled blanket under one hip to avoid supine hypotension (see p. 237). If she desires, the screen can be positioned so that she can see the images. Transmission gel is spread over her abdomen, and the sonographer, usually a physician or ultrasound technician, moves a transducer over the abdomen to obtain a picture (Figure 15-3). During the second trimester, a full bladder may be needed to displace the intestines and elevate the uterus for better visibility. If indicated, the woman should be instructed to drink several glasses of clear fluid an hour before the time of the examination and to delay urination until the examination is completed.

Purpose second trimester amniocentesis

The primary purpose for midtrimester amniocentesis is to examine fetal cells present in amniotic fluid to identify chromosomal or biochemical abnormalities. Amniocentesis is also used to evaluate the fetal condition when the woman is sensitized to Rh-positive blood, to diagnose amnionitis (intrauterine infection), and to test the AFAFP when MSAFP is abnormal and a cause for the abnormal levels cannot be determined by noninvasive tests

purpose of transvaginal ultrasound

Transvaginal ultrasound is most common during the first trimester for: •Determining the presence and location (intrauterine or elsewhere) of pregnancy •Detecting multifetal gestations •Estimating gestational age •Confirming fetal viability •Identifying the need for follow-up testing •Identifying ultrasound characteristics that suggest fetal abnormality, such as chromosome defects •As an adjunct for transcervical or transabdominal chorionic villus sampling During the first trimester, measurement of the crown-rump length of the embryo is the most reliable indicator of gestational age. Fetal viability is confirmed by observation of the fetal heartbeat, which is visible when the embryo is at least 5 mm in length. Maternal structures and some abnormalities such as uterine fibroids, ovarian cysts, and a bicornuate uterus also can be seen

transvaginal ultrasound procedure

Transvaginal ultrasound is often used during the first trimester because the uterus, gestational sac, embryo, ovaries, and fallopian tubes are deep in the pelvis.

multiple marker screening

Two other markers, human chorionic gonadotropin (hCG) and unconjugated estriol, have been added to routine MSAFP evaluation to screen for chromosomal abnormalities with maternal serum. This increases the detection of trisomy 18 and trisomy 21. Maternal serum samples are taken between 16 and 18 weeks of gestation, and the results are considered positive if MSAFP and estriol are low and if hCG is high.

placental hormone inhibin A

a fourth maker, improves the accuracy of multiple-marker screening for identifying trisomy 21 in women younger than the age of 35 years.

Amniocentesis

is aspiration of amniotic fluid from the amniotic sac for examination Amniocentesis may be performed during the second or third trimester of pregnancy, depending on the purpose. Second-trimester amniocentesis for fetal genetic abnormalities is best performed between 15 and 20 weeks because amniotic fluid volume is adequate and there are many viable fetal cells in the fluid. Early amniocentesis is possible between 11 and 14 weeks. Early amniocentesis is associated with a higher fetal loss rate than later amniocentesis. Fetal foot deformations are more likely to occur with the removal of amniotic fluid earlier than 13 weeks

Purpose of the doppler ultrasound blood flow assessment

pregnancies that have complications related to hypertension, may use one of these to look at the umbilical artery to check for any abnormalities in diastolic flow The most common measurement is the systolic/diastolic (S/D) ratio, which normally decreases throughout gestation. If fetal peripheral resistance rises, the diastolic flow falls, resulting in an increased S/D ratio. In severe cases of growth restriction caused by placental insufficiency, diastolic flow may be absent or even reversed

Procedure for AFP

pregnant women should be offered MSAFP screening, ideally between 16 and 18 weeks of gestation (AAP & ACOG, 2007). The mother is informed that MSAFP is a screening test rather than a diagnostic test. Further tests will be indicated to investigate abnormal concentrations. If MSAFP levels are abnormal, ultrasound is recommended initially to determine whether the abnormal concentration is caused by multifetal gestation, inaccurate gestational age, or fetal death.

Chronic Villus Sampling

Chorionic villi are microscopic projections from the outer membrane (chorion) that develop and burrow into endometrial tissue as the placenta is formed. The villi are composed of rapidly dividing cells of fetal origin that reflect the chromosomal and genetic makeup of the fetus. Chorionic villus cells can be used for diagnosis of fetal chromosomal, metabolic, or DNA abnormalities between 10 and 13 weeks of gestation.

Advantage of MSAFP evaluation

•It is a simple procedure that requires only a sample of maternal blood. •It is the least invasive and most economic procedure to screen for an open body wall defect such as neural tube defect or for chromosome abnormality. •Prenatal diagnosis allows parents time to examine their options or to prepare for the birth of an infant who will need special care.

Box 15-3 common indications for second trimester amniocentesis

• Maternal age 35 years or older • Chromosomal abnormality in close family member • Gender determination for maternal carrier of X-linked disorder (e.g., hemophilia, Duchenne muscular dystrophy) • Birth of previous infant with chromosomal abnormality or a neural tube or body wall defect • Pregnancy after multiple spontaneous abortions • Elevated levels of maternal serum alpha-fetoprotein that remain unexplained • Maternal Rh sensitization of maternal Rh-negative blood to fetal Rh-positive blood

Limitations of Chronic Villus Sampling

•The pregnancy loss rate is approximately 2.5% (Wapner et al., 2009). •Reports of a higher-than-expected rate of limb reduction defects has occurred in CVS performed before 10 weeks. Although CVS is now performed at 10 to 13 weeks, families should be given information about reported problems before they choose the procedure. •The risk for uterine infection is low, but it occasionally occurs. Presence of cervical or vaginal infection is a contraindication for the transvaginal approach (Cunningham et al., 2010; Gilbert, 2011). •Rh sensitization may occur as a result of entry of fetal Rh-positive blood cells into the circulation of an Rh-negative mother. Rho(D) immune globulin (RhoGAM) should be administered to all unsensitized Rh-negative women following the procedure (see Chapter 26). •CVS is labor intensive because maternal cells may be aspirated with the fetal cells. Maternal cells must be removed from the sample before culture, adding to the procedure's cost.