Fetal growth assessment by sonography





Objectives


On completion of this chapter, you should be able to:




  • Describe how intrauterine growth restriction may be detected by sonography



  • Differentiate between symmetric and asymmetric intrauterine growth restriction



  • List which growth parameters should be used to assess intrauterine growth restriction



  • Describe how to assess amniotic fluid volume



  • Describe how to perform a biophysical profile on a fetus



  • Discuss quantitative and qualitative Doppler measurements as applied to obstetrics



  • Analyze the significance of macrosomia in a fetus



  • Discuss the multiple fetal parameters and calculated ages used to assess the fetal somatic proportions and growth





Fetal growth assessment is very important to the perinatologist and obstetric physician. Before the availability of sonographic determination of fetal growth, physicians had to rely on their physical assessment of the neonate to determine what occurred during fetal development. The neonate’s assessment would determine whether the fetus was born preterm (before 37 menstrual weeks), early term (between 37 and 39 menstrual weeks), full term (between 39 and 41 menstrual weeks), later term (between 41 and 42 menstrual weeks), or postterm (after 42 weeks) (see Box 53-1 ). Contemporary fetal growth assessment by early sonographic dating and subsequent growth series examinations are most accurate. Further classification dictated whether the fetal birth weight was small for gestational age (SGA), intrauterine growth restricted, appropriate for gestational age, or large for gestational age (LGA). This determination allowed clinicians to recognize the increase in perinatal morbidity and mortality rates for preterm, postterm, intrauterine growth restricted, or LGA fetuses.



BOX 53-1

Maternal Factors for Intrauterine Growth Restriction (IUGR)





  • Previous history of fetus with IUGR



  • Significant maternal hypertension



  • History of tobacco use



  • Presence of uterine anomaly



  • Significant placental hemorrhage



  • Placental insufficiency






Intrauterine growth restriction


Intrauterine growth restriction (IUGR) is best described as a decreased rate of fetal growth. IUGR complicates 3% to 7% of all pregnancies. It is most commonly defined as a fetal weight at or below 10% for a given gestational age. It often becomes difficult to differentiate the fetus that is constitutionally small (relatively normal but SGA) from one that is growth restricted. IUGR (unwell) babies are at a greater risk of antepartum death, perinatal asphyxia, neonatal morbidity, and later developmental problems. Mortality rate is increased sixfold to tenfold, depending on the severity of the condition.


The most significant maternal factors for IUGR are the history of a previous fetus with IUGR, significant maternal hypertension or smoking, the presence of a uterine anomaly (bicornuate uterus or large leiomyoma), and significant placental hemorrhage ( Box 53-1 ). Constitutional factors such as the gender of the fetus, race of the mother, parity, body mass index, and environmental factors can affect the distribution of normal birth weight in any population.


Before abnormal growth can be diagnosed, the gestational age of the pregnancy must be accurately determined. In the prenatal period, an accurate last menstrual period or a first-trimester sonographic age can be used, and both are important for comparison to subsequent sonographic studies for growth assessment. If first-trimester sonography was not performed, in the second or third trimester the standard biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL), and other fetal parameters should be used in conjunction with other tests of fetal well-being (e.g., biophysical profile [BPP] and fetal Doppler velocimetry).


In the postnatal period, several other body dimensions can be used. These include head circumference, crown-heel length, weight-height ratios, ponderal index, and skinfold thickness. Other considerations include maternal size and race and the gender of the neonate. The standards for fetal growth have been incorporated with sonographic computer software, and different programs exist that can be applied to a variety of geographic locations.


The reader should not confuse IUGR with SGA. SGA describes the fetus with a weight below the 10th percentile without reference to the cause. Fetal growth restriction describes a subset of the SGA fetuses with a weight below the 10th percentile as a result of pathologic processes resulting from a variety of maternal, fetal, or placental disorders. The classification of IUGR is based on the morphologic characteristics of the fetuses studied. There are two basic clarifications: symmetric and asymmetric IUGR. The SGA fetus may simply be normal, but constitutionally small, but a fetus with IUGR is ill and does not follow a normal growth trajectory. IUGR is often progressive, with the fetal size lagging farther and farther behind the expected growth rate.


Symmetric IUGR is usually the result of a first-trimester insult, such as a chromosomal abnormality or infection. This results in a fetus that is proportionately small throughout the pregnancy. Approximately 20% to 30% of all IUGR cases are symmetric. The timing of the pathologic insult is recognized as more important than the actual nature of the underlying pathologic process.


Asymmetric IUGR generally begins in the second or third trimester and usually results from placental insufficiency. This fetus usually shows head sparing at the expense of abdominal and soft tissue growth. The fetal FL exhibits varying degrees of compromise. An early diagnosis of IUGR and close fetal monitoring (BPP, Doppler, and fetal growth evaluation) are of significant help in managing a pregnancy suspected of IUGR. Clinical observations and appropriate actions for IUGR are listed in Box 53-2 .



BOX 53-2

Clinical Observations and Actions for Intrauterine Growth Restriction (IUGR)





  • Clinical signs. Decreased fundal height and fetal motion.



  • Key sonographic markers. Grade 3 placenta before 36 weeks or decreased placental thickness.



  • Sonographer action. Alert the physician, determine the cause (maternal history, habits, environmental exposure, viruses, diseases, drug exposure), and carefully evaluate placenta and fetal anatomy with sonography.



  • Assessment of umbilical artery Doppler for increased resistance to flow. S/D greater than 3.0 after 30 LMP weeks is considered abnormal.




Symmetric intrauterine growth restriction


Symmetric growth restriction is characterized by a fetus that is small in all physical parameters (e.g., BPD, HC, AC, and FL), which is usually the result of a severe insult in the first trimester. The causes may include low genetic growth potential, intrauterine infection, severe maternal malnutrition, fetal alcohol syndrome, chromosomal anomaly, or severe congenital anomaly. One study demonstrated early IUGR in 9 of 11 fetuses with trisomy 13, 2 of 5 fetuses with 45 XO, and only 2 of 18 fetuses with trisomy 17. Another report described two cases of triploidy in association with early symmetric IUGR. Because of the increased association of chromosomal abnormalities, prenatal testing to rule out aneuploidy should be considered. Symmetric IUGR cannot be diagnosed by a single sonographic study because all parameters will have similar growth restriction. To diagnose symmetric IUGR takes two or more growth series over time (a month or more between examinations) so the fetal growth trajectory can be compared and analyzed ( Table 52-1 ).


Asymmetric intrauterine growth restriction


Asymmetric growth restriction is the more common form of IUGR and is usually caused by placental insufficiency. This may be the result of maternal disease, such as diabetes (classes D to F), chronic hypertension, cardiac or renal disease, abruptio placentae, multiple pregnancy, smoking, poor weight gain, drug usage, or uterine anomaly. It should be noted that IUGR fetuses have been born to mothers who have no high-risk factors; therefore all pregnancies undergoing sonographic examinations should be evaluated for IUGR.


Asymmetric IUGR is characterized by an appropriate BPD and HC and a disproportionately small AC. This reinforces the brain-sparing effect, which states that the last organ to be deprived of essential nutrients is the brain. The BPD and HC may be slightly smaller, but this usually does not happen until the late third trimester.


A proposed third type of IUGR suggests that fetuses with long FL (90th percentile or above) and small AC (at or below the 5th percentile) may be nutritionally deprived even though their estimated fetal weight (EFW) falls at least in the lowest 10%. The theory is that in asymmetric IUGR the fetal length is well preserved, whereas the soft tissue mass is deprived. The FL to AC ratio or ponderal index would be abnormally low. The proponents of this theory claim this occurs in less than 1% of IUGR cases but stress the importance of detection because these cases of IUGR have an EFW within the limits of normal.




Diagnostic criteria


The IUGR multiple parameters are shown in Box 53-3 .



BOX 53-3

Multiple Parameters for Intrauterine Growth Restriction (IUGR)





  • BPD. Imaged in the transverse plane using the cavum septi pellucidi, thalamic nuclei, falx cerebri, and choroid plexus as landmarks. The BPD can be misleading in cases associated with unusual head shapes. Used alone, it is a poor indicator of IUGR.



  • HC to AC ratio. High false-positive rate for use in screening general population. The HC to AC ratio is useful in determining the type of IUGR.



  • FL to AC ratio. Not dependent on knowing gestational age. The FL to AC ratio has a poor positive predictive value.



  • FL. May decrease in size with symmetric IUGR.



  • AC. Measure at level of portal-umbilical venous complex. When growth is compromised, AC is affected secondary to reduced adipose tissue and depletion of glycogen storage in liver. AC is the single most sensitive indicator of IUGR.




Biparietal diameter


The BPD is not a reliable predictor of IUGR for many reasons. The first is the head-sparing theory, which is associated with asymmetric IUGR. Fetal blood is shunted away from other vital organs to nourish the fetal brain, giving the fetus an appropriate BPD (plus or minus 1 standard deviation) for the true gestational age.


The second problem is the potential alteration in fetal head shape secondary to oligohydramnios. Oligohydramnios is a decreased amount of amniotic fluid often associated with IUGR. Dolichocephaly, or a falsely shortened BPD, can lead to underestimation of the fetal weight, and brachycephaly, or a falsely widened BPD, can lead to overestimation of the EFW. The HC measurement is a more consistent parameter, but a combination of multiple growth parameters (BPD, HC, AC, and FL) at a minimum should be used when diagnosing a fetal growth discrepancy.


Abdominal circumference


Because of the variability of fetal proportion and size, the AC is a poor predictor of gestational age but is valuable for assessing fetal size. In IUGR, the fetal liver is one of the most severely affected body organs, which therefore alters the circumference of the fetal abdomen.


Head circumference to abdominal circumference ratio


The HC to AC ratio was first developed to detect IUGR in cases of uteroplacental insufficiency. The HC to AC ratio is especially useful in differentiating symmetric and asymmetric IUGR. For each gestational age, a ratio is assigned with standard deviations. In an appropriate-for-gestational-age (AGA) pregnancy, the ratio should decrease as the gestational age increases.


In the presence of IUGR and with the loss of subcutaneous tissue and fat, the ratio increases. This is counterintuitive because as the fetal AC decreases, the HC:AC ratio increases, and vice versa. The HC to AC ratio is at least 2 standard deviations above the mean in approximately 70% of fetuses affected with asymmetric IUGR. The HC to AC ratio is not very useful, however, in predicting symmetric IUGR, because the fetal head and fetal abdomen are both equally small. This can be further complicated in cases of fetal infections (TORCH infections), which can produce organomegaly with enlargement of the liver or spleen and the resulting increase in the abdominal circumference in the presence of fetal IUGR.


Estimated fetal weight


The most reliable estimated fetal weight (EFW) formulas incorporate multiple fetal parameters, such as BPD, HC, AC, and FL. This is important because an overall reduction in the size and mass of these parameters naturally gives a below-normal EFW. An EFW below the 10th percentile is considered by most to be IUGR.


There are numerous formulas for estimating fetal weights. One method uses the BPD and AC to derive the fetal weight, with an accuracy of plus or minus 20%. This formula does not take into consideration HC and FL, which contribute to fetal mass. It also ignores the fact that BPD can be altered slightly because of normal variations in head shape, such as brachycephaly or dolichocephaly. These variations can occur in association with oligohydramnios, which may be found with IUGR.


Another method uses three basic measurements: HC, AC, and FL. The use of the HC instead of the BPD has improved the predictive value to plus or minus 15%.


A third method defines three zones of EFW. Each zone has a different prevalence of IUGR. In zone 1, the EFW is above the lower 20% confidence limit and IUGR is ruled out. In zone 3, the EFW is below the lower 0.5% confidence limit and yields an 82% prevalence of IUGR. Patients in this zone should be delivered as soon as lung maturity can be proven. If the EFW is between zone 1 and zone 3, it falls into zone 2, which has a 24% prevalence of IUGR. Patients in this zone should have serial sonograms and fetal heart rate monitoring.


Numerous other growth curves are available, but the one chosen must be appropriate for the population of patients (e.g., sea level versus above sea level). It is also important to remember that symmetric IUGR cannot be diagnosed in a single examination. The interval growth can be plotted on a graph or chart to show the growth sequences. Ethnicity, previous obstetric history, paternal size, fetal gender, and the results of tests of fetal well-being must be considered before IUGR, rather than a healthy SGA, can be diagnosed.


A computer-generated antenatal chart is available that can be customized for individual pregnancies, taking the mother’s characteristics and birth weights from previous pregnancies into consideration. Through review of 4179 pregnancies with sonographically confirmed dates, one study showed that in addition to gestation and gender, maternal weight at first antenatal visit, height, ethnic group, and parity were significant determinants of birth weight in the study population. Correction factors were calculated and entered into a computer program to adjust the normal birth weight percentile limits. With adjusted percentiles, the researchers found that 28% of babies that conventionally fit the criteria for SGA (less than the 10%) and 22% of those who were LGA (greater than the 90%) were in fact within normal limits for the pregnancy. Conversely, 24% and 26% of babies identified as small or large, respectively, with adjusted percentiles were missed by conventional unadjusted percentile assessment.

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May 29, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Fetal growth assessment by sonography
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