Ultrasonography
Ultrasonography is the primary imaging modality in the evaluation of the fetus because of its wide availability, quality of images, low cost, and lack of known adverse effects (Fig. 7-9). Placental and fetal size, multiple births, abnormalities of placental shape, and abnormal presenta- tions can also be determined. Many developmental anom- alies can also be detected prenatally by ultrasonography.
Diagnostic Amniocentesis
Diagnostic amniocentesis is a common invasive prenatal diagnostic procedure (Fig. 7-10A) typically performed during the second trimester. For prenatal diagnosis, amni- otic fluid is sampled by insertion of a hollow needle through the mother’s anterior abdominal and uterine walls and into the amniotic sac. A syringe is then attached to the needle and amniotic fluid is withdrawn. The Many factors—maternal, fetal, and environmental—
may affect prenatal growth. In general, factors operating throughout pregnancy, such as cigarette smoking and consumption of alcohol, tend to produce intrauterine growth restriction (IUGR) and small neonates, whereas factors operating during the last trimester (e.g., maternal malnutrition) usually produce underweight neonates with normal length and head size. Severe maternal malnutri- tion resulting from a poor-quality diet is known to cause reduced fetal growth (see Fig. 7-7).
Neonates (newborns) resulting from twin, triplet, and other multiple pregnancies usually weigh considerably less than infants resulting from a single pregnancy (see Fig. 7-7). It is evident that the total requirements of two or more fetuses exceed the nutritional supply available from the placenta during the third trimester.
Repeated cases of IUGR in one family indicate that recessive genes may be the cause of the abnormal growth.
In recent years, structural and numeric chromosomal aberrations have also been shown to be associated with cases of restricted fetal growth. IUGR is pronounced in neonates with trisomy 21 (Down syndrome) (see Chapter 19).
Figure 7–7 Graph showing the rate of fetal growth during the last trimester. After 36 weeks, the average growth rate deviates from the straight line. The decline, particularly after full term (38 weeks), probably reflects inadequate fetal nutrition caused by placental changes. Other factors affecting fetal growth rate (smoking, maternal malnutrition, twins) are also shown. (Modified from Gruenwald P: Growth of the human fetus. I. Normal growth and its variation. Am J Obstet Gynecol 94:1112, 1966.)
Average Smokers
Poor maternal nutrition Twins
3500
3000
2500
2000
1500
Weight (g)
Weeks from fertilization
26 30 34 38
1000
Figure 7–8 Full-term female neonate weighing 3.3 kg (7.2 lb).
Note the fatty vernix caseosa covering part of her body.
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 69
slightly more than the risk associated with amniocentesis (0.5%). The major advantage of chorionic villus sam- pling over amniocentesis is that it allows fetal chromo- somal sampling to be performed several weeks earlier.
Cell Cultures
Fetal sex and chromosomal aberrations can also be deter- mined by studying the sex chromosomes in cultured fetal cells obtained during amniocentesis. The cultures are commonly performed when an autosomal abnormality, such as occurs in Down syndrome, is suspected. Inborn errors of metabolism and enzyme deficiencies in fetuses can also be detected by studying cell cultures.
Percutaneous Umbilical Cord Blood Sampling
For chromosomal analysis, blood samples may be obtained from the umbilical vein by percutaneous umbili- cal cord blood sampling (PUBS). Ultrasonographic scan- ning is used to outline the location of the vessels. PUBS is often performed approximately 20 weeks after the LNMP to obtain samples for chromosomal analysis when ultrasound or other examinations have shown character- istics of birth defects.
Magnetic Resonance Imaging
When fetal treatment, such as surgery, is planned, com- puted tomography and magnetic resonance imaging (MRI) may be used. MRI has the advantage of not requir- ing ionizing radiation to produce images. These studies procedure is relatively devoid of risk, especially when
performed by an experienced physician using ultrasonog- raphy as a guide for outlining the position of the fetus and placenta.
Chorionic Villus Sampling
Biopsy of chorionic villi (see Fig. 7-10B) is performed to detect chromosomal abnormalities, inborn errors of metabolism, and X-linked disorders. Chorionic villus sampling can be performed as early as 7 weeks after fertilization. The rate of fetal loss is approximately 1%,
Figure 7–9 Ultrasonogram (axial scan) of a 25-week fetus showing the facial profile.
Figure 7–10 A, Illustration of the technique of amniocentesis. Using ultrasonographic guid- ance, a needle is inserted through the mother’s abdominal and uterine walls into the amniotic sac. A syringe is attached and amniotic fluid is withdrawn for diagnostic purposes. B, Illustration of chorionic villus sampling. Two sampling approaches are shown—one through the anterior abdominal wall and amniotic sac using a needle, and one through the vagina and cervical canal using a malleable chorionic villus catheter.
Placenta
Bladder
Vagina
Chorionic villus
A B
Syringe Ultrasound
transducer
Ultrasound transducer
Bladder Uterine wall
Amniotic sac Speculum
Chorionic villus catheter
C H A P T E R 7 FETAL PERIOD: THE NINTH WEEk TO BIRTH 69.e1
(Courtesy E. A. Lyons, MD, Professor of Radiology, Obstetrics and Gynecology, and Anatomy, University of Manitoba, Health Sci- ences Centre, Winnipeg, Manitoba, Canada.)
70 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
The neonate (newborn) is not a miniature adult, and an extremely preterm infant is not the same as full-term infant. The late neonatal period is from 7 to 28 days.
The umbilical cord usually drops off 7 to 8 days after birth, the end of the early neonatal period.
At birth, the head of a neonate is large in proportion to the rest of its body. Thereafter, the head grows more slowly than the trunk (torso) of the body. Usually a neonate loses about 10% of its birth weight 3 to 4 days after birth, owing to the loss of excess extracellular fluid and discharge of meconium, the first greenish intestinal material ejected from the rectum.
When the neonate’s hand is touched, it will usually grasp a finger. If the mother holds the baby close to her chest, the baby will search (root) for her breast to find the nipple and feed. Neonates are born with full visual capacity to see objects and colors about 8 to 15 inches away; however, they are extremely nearsighted. Some preterm neonates’ eyes are crossed because the eye muscles are not fully developed. A gentle stroke on the baby’s cheek makes the baby turn toward the touch with its mouth open.
CLINICALLY ORIENTED QUESTIONS 1. Do mature embryos move at all? Can a first-trimester
fetus move its limbs? If so, can the mother feel her baby kicking at this time?
2. Some reports suggest that vitamin supplementation around the time of conception will prevent neural tube defects, such as spina bifida. Is there scientific proof to support this statement?
3. Can the needle injure the fetus during amniocentesis?
Is there a risk of inducing an abortion or causing maternal or fetal infection?
The answers to these questions are at the back of this book.
can provide additional information about a fetal abnor- mality detected by ultrasound.
Fetal Monitoring
Continuous fetal heart rate monitoring in high-risk preg- nancies is routine and provides information about the oxygenation of the fetus. Fetal distress, as indicated by an abnormal heart rate or rhythm, suggests that the fetus is in jeopardy.
Alpha-Fetoprotein Assay
Alpha fetoprotein (AFP), a glycoprotein that is synthe- sized in the fetal liver and umbilical vesicle, escapes from the fetal circulation into the amniotic fluid in fetuses with an open neural tube defects, such as spina bifida with myeloschisis (see Chapter 19). AFP can also enter the amniotic fluid from open ventral wall defects, as occurs with gastroschisis and omphalocele (see Chapter 13).
AFP can also be measured in maternal serum.
Noninvasive Prenatal Diagnosis
Down syndrome (trisomy 21) is the most commonly known of the chromosomal disorders, and children born with this condition have varying degrees of intel- lectual disability. Noninvasive screening for trisomy 21 is based on the isolation of fetal cells in maternal blood and the detection of cell-free fetal DNA and RNA. Com- pared to amniocentesis and chorionic villus biopsy, the results are available earlier and there are fewer complica- tions. The methodology of this DNA-based diagnostic test continues to evolve and be refined to improve its reliability.