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 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

Key Terms

Biophysical profile (BPP)

Continuous wave (CW) Doppler

Estimated fetal weight (EFW)

Intrauterine growth restriction (IUGR)

Large for gestational age (LGA)

Macrosomia

Nonstress test (NST)

Pulsatility index (PI)

Pulsed wave (PW) Doppler

Resistance index (RI)

Small for gestational age (SGA)

Systolic to diastolic (S/D) ratio

Fetal growth assessment is very important to the perinatologist and obstetric physician. Prior to the 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 if the fetus was born preterm (35 to 36 weeks), early term (37 to 38 weeks), full term (39 to 40 weeks), late term (41 weeks), or postterm (after 42 weeks). Clinicians now utilize ultrasound to assess accurate fetal growth by early sonographic dating and subsequent growth series examinations.

Further classification of fetal growth dictated whether the fetal birth weight was appropriate for gestational age, small for gestational age, or large for gestational age or demonstrated restricted intrauterine growth. Fetal growth restriction describes a fetus that is abnormally small for gestational age, which is often the result of placental insufficiency. This chapter covers growth restriction and the fetal monitoring tests available for the growth restricted fetus, which include the nonstress test, biophysical profile, and umbilical artery Doppler evaluation.

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 small for gestational age [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 ( Box 53.1 ).

Box 53.1

Key Points for IUGR

IUGR , Intrauterine growth restriction.

Oligohydramnios occurs if the fetal urine output is reduced.
Polyhydramnios develops if the fetus cannot swallow.
Amniotic fluid pocket <1–2 cm may represent IUGR.
Not all oligohydramnios is associated with IUGR.

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 53.2 ). Constitutional factors such as the sex 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.

Box 53.2

Maternal Factors for Intrauterine Growth Restriction

Previous history of fetus with intrauterine growth restriction
Significant maternal hypertension
History of tobacco use
Presence of uterine anomaly
Significant placental hemorrhage
Placental insufficiency

Before abnormal growth can be diagnosed, the gestational age of the pregnancy must be accurately determined, as presented in Chapter 52 . 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, then in the second or third trimester, the standard biparietal diameter, head circumference, abdominal circumference, 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 Box 53.3 ).

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 not following a normal growth trajectory. IUGR is often progressive, with the fetal size lagging further and further 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 53.4 .

Symmetric Intrauterine Growth Restriction

Symmetric growth restriction is characterized by a fetus that is small in all physical parameters (e.g., biparietal diameter [BPD], head circumference [HC], abdominal circumference [AC], and femur length [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. 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 (at least one month 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, 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.

Small for Gestational Age

Ultrasound is utilized to discover if a fetus is smaller than normal for their gestational age. The term small for gestational age (SGA) is used to describe a neonate or fetus whose birth weight falls below the 10th percentile. The small fetus has been shown to have a compromised fetal growth. The causes for a SGA fetus may include genetic diseases, inherited metabolic disease, chromosome anomalies, or multiple gestations (twins, triplets, and more). The growth restricted pregnancy has an increased risk of perinatal morbidity and mortality, including fetal demise, brain injury, fetal distress, neonatal hypothermia, hypoglycemia, hyperbilirubinemia, and decreased immune function.

Multiple parameters are recommended to determine if the fetus is at risk for growth restriction. These parameters include EFW percentile, amniotic fluid volume, and maternal blood pressure status (normal vs. hypertensive). Benson and Doubilet state that for each week of data on gestation, amniotic fluid volume classification, and maternal blood pressure status, the table presents an EFW range. For a particular fetus, if the EFW is below the lower end of the EFW range, then the fetus can be confidently diagnosed with growth retardation. If the EFW is above the upper end of the range, growth retardation can be excluded with confidence. If the EFW falls within the range, the likelihood of growth retardation is elevated but indeterminate. Doppler of the umbilical and uterine arteries is useful to monitor changes and guide management decisions.

Close fetal monitoring is necessary for the small growth restricted fetus, which includes the nonstress test, the BPP, and the umbilical artery Doppler evaluation.

Diagnostic Criteria

The multiple parameters of IUGR are shown in Box 53.3 . Key points of IUGR are listed in Box 53.1 .

Box 53.3

Multiple Parameters for IUGR

AC , Abdominal circumference; BPD , biparietal diameter; FL , femur length; HC , head circumference; IUGR , intrauterine growth restriction.

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:AC ratio: high false-positive rate for use in screening general population. HC:AC ratio is useful in determining the type of IUGR.
FL:AC ratio: not dependent on knowing gestational age. FL: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.

Box 53.4

Clinical Observations and Actions for Intrauterine Growth Restriction

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: systolic to diastolic ratio >3.0 after 30 weeks after last normal menstrual period is considered abnormal.

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 (±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.

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