Size Less than Dates

• Discuss the clinical scenarios associated with a pregnancy size less than dates.

• Describe the sonographic appearance and technique for a diagnosis of oligohydramnios.

• Describe the sonographic appearance of common fetal renal diseases associated with size less than dates.

• Describe clinical conditions, basic physiology, and sonographic findings to understand and make a diagnosis of intrauterine growth restriction.

• Discuss the role of sonography in the diagnosis of premature rupture of membranes.

CLINICAL SCENARIO

A 39-year-old woman, gravida 1, para 0, is seen at the high-risk obstetric clinic for a sonogram of the fetus. She is a professional in a high-stress career who works 70 to 80 hours a week. On her last prenatal visit, the uterine fundal height measured 30 cm and lagged behind the gestational age of 33 weeks, 4 days. She had a prior sonographic examination at 16 weeks that confirmed the gestational age was consistent with her last menstrual period. The prior sonogram also failed to reveal any obvious fetal congenital anomalies, and a genetic amniocentesis showed normal fetal chromosomes. Her medical history is significant for type 1 diabetes mellitus since age 12 years; her condition is well controlled with an insulin pump. During her pregnancy, blood pressures have been normal and she has always had a trace of proteinuria.

The current sonogram reveals a single viable fetus in the vertex presentation. Fetal biometry shows an average ultrasound age of 31 weeks, 5 days, and an amniotic fluid index (AFI) of 6.7 cm. The estimated fetal weight is at the 8th percentile for gestational age. The placenta is posterior-fundal and unremarkable. Clinically, her blood pressure is 149/100 mm Hg, and her urine dipstick test reveals significant protein. She also has a severe headache. What are the possible explanations for these findings?

Measurement of the uterine fundal height during every prenatal visit after 20 weeks of gestation is an integral part of obstetric care. It is a simple, safe, inexpensive, and reasonably accurate method to screen for fetal growth and amniotic fluid volume abnormalities, although it is less accurate in identifying small-for-gestational-age (SGA) fetuses. Longitudinal studies have shown that symphysis-to-fundus measurements correctly identify less than 35% of SGA fetuses.¹ The standard of care is to order or perform an obstetric sonogram when the uterine fundal height is in discordance with the estimated gestational age.

When evaluating a woman considered to have a pregnancy size less than dates, obtaining a medical history, vital signs, social and family history, and recent symptoms is warranted. Some patients may have decreased fetal movement, which would suggest the finding of oligohydramnios. Others may be found to have recently elevated blood pressure or a history of vascular disease, such as pregestational diabetes mellitus or systemic lupus erythematosus. Family history may reveal prior cases of neonates with renal abnormalities or a tendency for small infants. Evaluation of social habits, such as work, nutrition, and use of alcohol or recreational drugs, can be useful. Generally, a carefully performed obstetric sonogram together with pertinent clinical data is more likely to yield an accurate diagnosis.

Oligohydramnios

Oligohydramnios is a significant decrease in the normal volume of amniotic fluid. Amniotic fluid volume (AFV) results from a balance between what enters and exits from the amniotic cavity. From 20 weeks of gestation to term, the amniotic fluid is mostly made up of fetal urination and respiratory secretions, and fluid is resorbed via fetal swallowing. Alteration of these dynamics can have a significant impact on the pregnancy. The functions of amniotic fluid are numerous, including preventing fetal injury, regulating temperature, providing mobility for practicing breathing, swallowing exercises, fighting infection, discouraging contractions, and maintaining cervical length and consistency. A decrease in AFV is directly correlated with perinatal mortality and many serious morbidities.²

Oligohydramnios or anhydramnios (absence of amniotic fluid) can result from amniotic fluid loss from premature rupture of membranes (PROM) (Fig. 20-1) or can be a result of decreased fetal urinary production or excretion. Decreased urine output is associated with fetal renal anomalies in which both kidneys are dysplastic and with severe intrauterine growth restriction (IUGR) when perfusion to the kidneys is reduced. Decreased excretion is associated with urinary outlet obstruction. Ruptured membranes may be suggested with the finding of decreased amniotic fluid on sonography and a fetus that is of an appropriate size without structural anomalies. Fetal anomalies, medication or drug use by the mother (nonsteroidal antiinflammatory drugs, angiotensin-converting enzyme inhibitors, cocaine), maternal medical disease, placental insufficiency, and chromosome alterations all can explain a finding of oligohydramnios. Chronic oligohydramnios, with or without fetal abnormalities, may cause pulmonary hypoplasia, abnormal chest wall compliance, contractures, and infection.²

FIGURE 20-1 Severe oligohydramnios in a fetus at gestational age 20 weeks, 3 days. Note lack of fluid around fetal head **(A)** and abdomen **(B)**. The fetus is in breech position. A single pocket of amniotic fluid measuring 0.9 cm is identified. This patient had reported abdominal pain and leaking fluid.

Sonographic Findings

Although the most accurate methods to determine total AFV are direct measurement during hysterotomy or dye-dilution techniques, sonographic estimation is the only practical method. Sonographic methods include qualitative assessment (subjective) and quantitative assessment (objective). Most clinicians use some form of objective measurement to estimate the AFV, such as the maximum vertical pocket (MVP) or AFI.

MVP involves a survey of the entire amniotic cavity and measurement of the largest vertical pocket of amniotic fluid, free of umbilical cord or fetal parts. The criteria for oligohydramnios vary, but a deepest pocket of less than 2 cm is most widely accepted.³

AFI is calculated by adding the largest vertical pocket of fluid measured from each of the four quadrants of the uterus. Measurement of pockets of fluid should be void of umbilical cord or any fetal parts. AFI should be measured with the patient supine and can be obtained in the transverse or sagittal plane orientating the transducer perpendicular to the floor. To divide the uterus into four quadrants, the midsagittal line is used as the vertical landmark, and an imaginary line halfway between the symphysis pubis and the fundus of the uterus is used as the horizontal landmark. Using excessive transducer pressure can cause an inaccurate AFI assessment. It is important to use color Doppler imaging while obtaining an AFI measurement (Fig. 20-2; see Color Plate 18) because this eliminates the inaccuracy of measuring the umbilical cord instead of the amniotic fluid, especially on patients who are difficult to scan. AFI of less than 5 cm is the generally accepted criterion used to diagnose oligohydramnios, and AFI between 5.1 cm and 18 cm is considered normal.³

FIGURE 20-2 Oligohydramnios is noted in a pregnancy at gestational age 19 weeks. Only 1.9 cm of fluid is noted around the fetus. Color Doppler imaging helps identify the umbilical cord.

The ability of the described sonographic methods to detect normal AFV is high; however, abnormal AFV detection is poor, especially for oligohydramnios. Studies have compared AFI and MVP, and results indicate that AFI overestimates and MVP underestimates fluid volumes.3,4

When the diagnosis of oligohydramnios is made, detailed sonographic examination of the fetus should follow; however, evaluation of other anatomic structures may be limited because of the lack of an acoustic window. Sonographic evaluation determines if the kidneys are present and functioning, and fetal biometry and Doppler assessment can evaluate for IUGR. When major anomalies are not present, onset, duration, and severity of amniotic fluid loss are important cofactors, but the gestational age at delivery remains the overriding issue.²

Fetal Renal Anomalies

Renal Agenesis

Renal agenesis is the congenital absence of one or both kidneys from the complete lack of formation. The incidence rate of renal agenesis is 1 in 3000 births and 1 in 240 stillbirths.5,6 It is 2.5 times more common in male fetuses than in female fetuses. Bilateral absence of the kidneys is more common in twins than in singletons. An increased incidence is not seen with advanced maternal age or maternal medical disease.

Renal agenesis is a developmental anomaly that occurs at 4 to 6 weeks of embryonic life. Three main embryologic events are necessary for normal formation of the kidneys. Failure of one of the steps in renal formation (the development of the metanephros) results in complete absence of the kidney. Because of the absence of fetal urination, oligohydramnios develops in a woman with a fetus with bilateral renal agenesis, which may be clinically identified as size less than dates. In unilateral renal agenesis, one of the kidneys does not develop, and the contralateral fetal kidney undergoes compensatory hypertrophy. This hypertrophy occurs prenatally. Because renal function exists in the presence of unilateral renal agenesis, the AFV is usually within normal limits.

Sonographic Findings

The sonographic criteria for the diagnosis of bilateral renal agenesis are (1) presence of severe oligohydramnios or anhydramnios after 14 to 16 weeks of gestation and (2) failure to visualize the fetal kidneys and the urinary bladder (Fig. 20-3, A; see Color Plate 19). After 26 to 28 weeks of gestation, preterm premature rupture of membranes (PPROM) or placental dysfunction may be the cause for the low or absent AFV.

FIGURE 20-3 **(A),** Renal agenesis was confirmed in this breech fetus with severe oligohydramnios. Neither bladder nor renal arteries were identified. **(B),** Renal agenesis in a 29-week fetus shows enlarged adrenal glands *(calipers)* occupying renal spaces. Oligohydramnios and an absent bladder confirmed the diagnosis. **(C),** Normal renal arteries are apparent. Color Doppler maps out renal arteries bilaterally in a growth-restricted fetus with oligohydramnios, confirming the presence of kidneys that are poorly visualized.

First-trimester diagnosis of renal agenesis is rare because AFV is often not reduced at that point in gestation because its volume is primarily from the placenta. The fetal kidneys may be visualized from 10 weeks of gestation with transvaginal sonography. In contrast to the fetal adrenals, which are relatively hypoechogenic, the fetal kidneys appear as bilateral echogenic masses with a similar density to that of the fetal lungs. The fetal adrenal glands have served as a source of false-negative diagnoses in cases of renal agenesis. They may appear enlarged and be confused for fetal kidneys. However, adrenal hypertrophy has been found not to occur, and the false-negative diagnosis of renal agenesis is related to a change in the normal adrenal shape and not to any enlargement of the fetal adrenal gland. In the absence of a fetal kidney, the adrenal gland, which is no longer in its transverse lie draped over the superior pole of the kidney, now is primarily in a longitudinal lie (Fig. 20-3, B). This position gives it an enlarged appearance within the renal fossa.

Difficulty may arise in identifying the fetal kidneys in the presence of oligohydramnios. Color or power Doppler imaging is recommended as an adjunct in the diagnosis of bilateral renal agenesis. With correct Doppler settings, color Doppler imaging can aid in identifying the renal arteries (Fig. 20-3, C; see Color Plate 20), and renal agenesis is strongly suspected when the renal arteries are not identified.

Autosomal Recessive Polycystic Kidney Disease

Autosomal recessive polycystic kidney disease (ARPKD) occurs in approximately 1 in 20,000 births.⁷ This type of polycystic kidney disease is caused by mutations in the PKHD1 gene and is characterized by nonobstructive dilations of the collecting ducts in the kidneys and hepatic fibrosis.⁸ Severe cases can result in perinatal death when pulmonary hypoplasia is caused by long-standing oligohydramnios. Both kidney and liver disease are progressive in most patients, requiring kidney transplantation before the age of 20 years or treatment of early-onset severe hypertension, esophageal varices, and recurrent cholangitis, or both transplantation and treatment of the aforementioned complications.⁸ Fetal liver changes may not be evident sonographically; however, 30% of patients with ARPKD present perinatally with enlarged kidneys.⁸ DNA previously collected from the first fetus affected with ARPKD is invaluable to facilitate early prenatal diagnosis in subsequent pregnancies.

Sonographic Findings

Typical in utero presentation of ARPKD is the finding of enlarged, echogenic kidneys with loss of corticomedullary differentiation, along with oligohydramnios (Fig. 20-4). The echogenicity of the renal parenchyma is thought to be the result of numerous, closely spaced small cysts that cannot be detected sonographically. Increased echogenicity has been identified by 12 to 16 weeks of gestation. An accurate diagnosis of ARPKD may be difficult early in pregnancy because the size of the fetal kidneys may still be normal. Evidence of renal enlargement and marked echogenicity is usually present by 24 weeks of gestation. Occasionally, the diagnosis cannot be made until the third trimester or after birth. Oligohydramnios is associated with renal dysfunction, and marked oligohydramnios is a common finding. A detailed family history, together with sonographic evaluation of the parents’ kidneys, is helpful in obtaining the correct diagnosis because both ARPKD and autosomal dominant polycystic kidney disease can manifest with oligohydramnios.