Prenatal ultrasound is an integral part of caring for pregnant women in the United States. Although surprisingly few data exist to support the clinical benefit of screening ultrasound during pregnancy, its use continues to rise. Urologic anomalies are among the most commonly identified, with overall detection sensitivity approaching 90%. Prenatal hydronephrosis is the most frequently identified finding and predicting postnatal pathology based on its presence can be difficult. As the degree of fetal hydronephrosis increases so does the risk of true urinary tract pathology. Diagnoses that require more urgent care include causes of lower urinary tract obstruction and bladder and cloacal exstrophy.
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Fetal ultrasound is a routine part of prenatal care in the United States despite limited evidence of clinical benefit.
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Prenatal ultrasound use is rising in North America and urologic anomalies are among the most commonly detected findings.
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Hydronephrosis is the most frequently identified fetal urologic abnormality but the severity and clinical implications of prenatal hydronephrosis can vary greatly. As the severity of hydronephrosis increases so does the risk for clinically significant urinary tract pathology.
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The majority of fetuses with a urologic anomaly can be managed expectantly and only a small minority of fetuses will require urgent attention.
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Fetuses with suspected lower urinary tract obstruction comprise the group that may need urgent pediatric urology consultation and may even require fetal intervention.
Introduction
The performance of ultrasonography during pregnancy in the United States has become commonplace in the obstetric care of women. In reality, the practice of prenatal ultrasound exists largely outside the control of pediatricians or pediatric urologists. Despite the embracement of prenatal screening for organ system anomalies as routine by the medical community as a whole, the long-term clinical impact of identifying congenital anomalies prenatally remains undefined for many diagnoses. For some anomalies, inconsistent correlation between prenatal ultrasound appearance and postnatal clinical outcome leads to uncertainty in the aggressiveness with which postnatal evaluation should be pursued. By contrast, select anomalies may benefit from prenatal diagnosis by allowing for prompt and immediate tertiary care after birth or by providing the opportunity for fetal intervention before delivery.
For practicing pediatricians, a common clinical scenario likely exists. A newborn with a prenatally diagnosed urologic finding is now under the care of a pediatrician in the newborn nursery. Several questions likely come to mind. How much information was obtained about this anomaly before delivery? Should more or less information have been acquired before birth? Did the parents receive counseling from a pediatric urologist during gestation? How should this anomaly classified? Is it mild or is it severe? What are the next steps in the care of the neonate?
The severity of congenital urologic anomalies can be highly variable. In some children, the correct diagnosis and subsequent course of action is clear from the start, yet in other patients, such decisions may be less obvious. This review hopes to provide a clear reference for pediatricians as they see newborn babies with prenatally diagnosed urologic issues in their practice. The complexities of the postnatal evaluation in neonates with a urologic anomaly, specifically PNH, are beyond the scope of this article and are not addressed.
Ultrasound use in North America
Few medical technologies have had such rapid incorporation into the care of patients as has prenatal ultrasonography. Although some parents incorrectly view the early second-trimester ultrasound as an opportunity to diagnose fetal gender, its purpose is to screen for organ system anomalies. Prenatal ultrasound in obstetric care has its temporal roots in England, where, in the late 1950s, it was used for detecting abdominal masses in women. In the early 1960s, investigators in Glasgow began measuring fetal cephalic growth in gravid women. A 1970 report describing the prenatal diagnosis of polycystic kidneys was a seminal event in the prenatal identification of organ anomalies. The mainstream incorporation of sonographic fetal anomaly screening in obstetrics occurred as a result of several clinical trials conducted over the past 30 years. The ability to prove the clinical benefit of routine screening for organ anomalies remains elusive. A randomized trial from Europe compared prenatal ultrasound screening with expectant management and reported improved fetal survival after prenatal diagnosis of organ anomalies, but this survival improvement was highly influenced by pregnancy terminations that occurred after severe anomalies were detected. To date, the only randomized trial in the United States evaluating routine ultrasound screening during pregnancy failed to conclusively demonstrate that the use of prenatal ultrasound and subsequent prenatal anomaly diagnosis has a positive impact on clinical outcome. Furthermore, 2 separate meta-analyses were also unable to demonstrate that routine use of ultrasound in low-risk or unselected pregnant women leads to a reduction in adverse outcomes or provides clinical benefit.
Routine prenatal ultrasound use is on the rise. From 1995 to 2006 in the United States, the mean number of prenatal sonograms performed per pregnancy reportedly increased from 1.5 to 2.7. By the year 2006, women with high-risk pregnancies underwent twice as many studies, having an average of 4.2 ultrasounds per pregnancy. Similar US reports document an approximate doubling of prenatal ultrasounds performed over a 7-year period, from 1998 to 2005. Likewise, in Canada, a 55% increase in prenatal ultrasound use was recognized between 1996 and 2006.
Ultrasound use in North America
Few medical technologies have had such rapid incorporation into the care of patients as has prenatal ultrasonography. Although some parents incorrectly view the early second-trimester ultrasound as an opportunity to diagnose fetal gender, its purpose is to screen for organ system anomalies. Prenatal ultrasound in obstetric care has its temporal roots in England, where, in the late 1950s, it was used for detecting abdominal masses in women. In the early 1960s, investigators in Glasgow began measuring fetal cephalic growth in gravid women. A 1970 report describing the prenatal diagnosis of polycystic kidneys was a seminal event in the prenatal identification of organ anomalies. The mainstream incorporation of sonographic fetal anomaly screening in obstetrics occurred as a result of several clinical trials conducted over the past 30 years. The ability to prove the clinical benefit of routine screening for organ anomalies remains elusive. A randomized trial from Europe compared prenatal ultrasound screening with expectant management and reported improved fetal survival after prenatal diagnosis of organ anomalies, but this survival improvement was highly influenced by pregnancy terminations that occurred after severe anomalies were detected. To date, the only randomized trial in the United States evaluating routine ultrasound screening during pregnancy failed to conclusively demonstrate that the use of prenatal ultrasound and subsequent prenatal anomaly diagnosis has a positive impact on clinical outcome. Furthermore, 2 separate meta-analyses were also unable to demonstrate that routine use of ultrasound in low-risk or unselected pregnant women leads to a reduction in adverse outcomes or provides clinical benefit.
Routine prenatal ultrasound use is on the rise. From 1995 to 2006 in the United States, the mean number of prenatal sonograms performed per pregnancy reportedly increased from 1.5 to 2.7. By the year 2006, women with high-risk pregnancies underwent twice as many studies, having an average of 4.2 ultrasounds per pregnancy. Similar US reports document an approximate doubling of prenatal ultrasounds performed over a 7-year period, from 1998 to 2005. Likewise, in Canada, a 55% increase in prenatal ultrasound use was recognized between 1996 and 2006.
Screening sensitivity
The combination of routine prenatal ultrasound and improving ultrasound technology ensures that more organ anomalies are detected before birth. Perhaps the best available data regarding the frequency of organ system anomalies come from 2 separate large-scale ultrasound screening studies, the Eurofetus study and the EuroScan study, performed in obstetric centers throughout Europe in the 1990s. In both studies, approximately 2% of all pregnancies were affected by a congenital anomaly. The frequency of detecting anomalies on prenatal ultrasound is heavily contingent on the organ system studied and the experience of the center performing the study. In the Eurofetus study, a second-trimester screening ultrasound detected 61% of postnatally confirmed anomalies before birth, with 44% detected before 24 weeks. By contrast, in the only randomized trial of prenatal ultrasound screening ever performed in the United States, the Routine Antenatal Diagnostic Imaging with Ultrasound trial, the sensitivity for detecting organ system anomalies before 24 weeks of gestation was only 16% and was 35% irrespective of gestational age. The sensitivity for detecting urologic anomalies before birth seems universally high. Table 1 shows the relative distribution of anomalies by organ system identified at birth in the Eurofetus study. Of the 954 urogenital anomalies detected in Eurofetus, 88.5% were identified prenatally. By contrast, heart and great vessel anomalies were identified only 27% of the time.
| System | Total Number | Postnatal Frequency (%) | Prenatal Ultrasound Sensitivity (%) |
|---|---|---|---|
| Musculoskeletal anomalies | 1043 | 23 | 36 |
| Heart and great vessels | 953 | 21 | 27 |
| Urinary tract abnormalities | 954 | 21 | 88 |
| Central nervous system | 738 | 16 | 88 |
Normal fetal urinary tract appearance
Reviewing the normal appearance of the fetal urinary tract is a prerequisite to discussing congenital anomalies of genitourinary system. Fig. 1 depicts the normal ultrasound appearance of the fetal kidney and bladder on prenatal ultrasound. The healthy fetal kidney is typically not visualized on a transabdominal ultrasound until at least week 15 of gestation. Fetal renal length varies during development (see Fig. 1 ). The anterior posterior diameter (APD) of the fetal renal pelvis is a measurement that has been increasingly used to categorize fetal renal dilatation as normal or abnormal. Fetal renal APD measurements less than 4 mm in the second trimester and less than 7 mm in the third trimester are considered physiologic levels of fetal renal dilatation. In the normal fetus, the ureters are not visible on prenatal ultrasound whereas normal ureteral diameter in neonates is reported to be 5 mm or less. The fetal bladder is visible on transvaginal ultrasound in 87% of cases by 12 weeks of gestation and care should be taken to ensure the bladder is identified during the second-trimester screening ultrasound. If the bladder is not visualized, repeat imaging later in the study or on a subsequent repeat ultrasound should confirm its presence or absence. Bladder enlargement, also termed megacystis may be noted on prenatal ultrasound and can suggest urinary tract obstruction. Measurements that define a normal fetal bladder size have not been concretely defined. From weeks 10 to 14, suggested normal parameters for bladder size include either a longitudinal bladder diameter of less than 6 mm or a diameter measuring less than 10% of crown-rump length. Normal bladder size in the second and third trimesters remains undefined. A normal bladder in the second trimester has been characterized subjectively as one of small size that empties during a 45-minute time frame. Amniotic fluid levels can be a surrogate marker for fetal urinary tract function. Although fetal urine production begins by 8 to 10 weeks of gestation, it is only beyond 16 weeks of development that the amniotic fluid is primarily composed of fetal urine. Thus, abnormalities in the amniotic fluid levels in the second and third trimesters may be harbingers of urinary tract problems.
Prenatal urologic anomalies
A variety of urologic diagnoses may be detected before birth. The certainty of prenatal suspicion, however, can only be confirmed with accurate postnatal evaluation and diagnosis. A follow-up report from the EuroScan study detailed the diagnoses in 1130 patients with urologic anomalies diagnosed from a population of 709,030 births. Table 2 lists the most common diagnoses in the 609 patients with isolated urologic anomalies and the percentage of each diagnosis that were detected prenatally.
| Anomaly | Percentage of Total | Percentage Detected Prenatally |
|---|---|---|
| All anomalies (n = 609) | — | 82 |
| Hydronephrosis | 51 | 84 |
| Multicystic dysplastic kidney | 17 | 97 |
| Unilateral renal agenesis | 10 | 62 |
| Duplicated kidney | 6 | 95 |
| Renal ectopia | 4 | 56 |
| Posterior urethral valves | 4 | 70 |
| Solitary renal cyst | 4 | 76 |
| Bladder exstrophy | 3 | 53 |
The gestational age at which urologic anomalies are first identified is of variable importance. Almost universally, the presence of oligohydramnios in the second trimester is a poor prognostic sign for fetal survival. Fetal megacystis may be seen as early as the first trimester, and spontaneous resolution is reported common in fetuses with longitudinal diameters less than 12 mm. In the second and third trimesters, megacystis alone has little predictive value. Rather, the clinical picture of the fetus, from a urologic perspective, should be derived from the ultrasound appearance of the entire urinary tract and the amniotic fluid level, not simply the size of the bladder. The importance of timing in the initial identification of PNH (ie, second-trimester detection vs third-trimester detection) and its correlation with true urinary tract pathology is uncertain. With the majority of screening ultrasounds occurring during the second trimester, women with a normal second-trimester ultrasound are unlikely to undergo a repeat ultrasound in the third trimester. A few studies, however, have shown that PNH severity on a third trimester scan may be more predictive of clinical outcome than the appearance on a second-trimester study.
This discussion of specific prenatal urologic diagnoses begins with hydronephrosis. The remainder of the review focuses on those diagnoses that can be managed expectantly and those that require more urgent attention in the fetal and neonatal period.
Hydronephrosis
PNH is the most commonly identified prenatal urinary tract abnormality. Unfortunately, hydronephrosis is not actually a diagnosis but rather a sign of some other underlying problem in the urinary tract. A recent meta-analysis of 17 studies identified 1678 fetuses with PNH of a total screened population of 104, 572 (1.6% prevalence of PNH). Of these 1678, 36% had identifiable urinary tract pathology on postnatal evaluation. Several factors make the interpretation of PNH controversial, including the lack of diagnostic specificity, the variable methods for classifying its severity, and the inconsistent postnatal clinical outcomes with varying degrees of dilatation, particularly in children with mild or moderate PNH.
Hydronephrosis Grading
Accurately classifying the degree of upper urinary tract dilatation in the fetus and neonate can be difficult. Ideally, PNH would be graded using one objective scale that could then accurately predict the risk of true postnatal pathology. Fetuses would be stratified into prognosis groups accordingly, which help guide the postnatal evaluation of the fetus. Unfortunately, no one system to date conclusively allows for such determinations. In general, as the degree of PNH increases so does the risk for persistent postnatal pathology.
As discussed previously, the APD of the renal pelvis is a commonly used method for defining PNH. For proper APD calculations, measurements should be obtained from a transverse axial image of the renal pelvis at approximately the level of the renal hilum. Renal APD measurements can vary depending the gestational age of the fetus, and the thresholds for concern must change as well. An early report from George Washington University used renal APD threshold values of 4 mm before 33 weeks and 7 mm after 33 weeks to define hydronephrosis. The study found both of these gestational age–based APD thresholds nearly 100% sensitive for detecting PNH but poorly specific for predicting both the postnatal persistence of hydronephrosis and the need for postnatal surgery. Repeat analysis of the data in a separate report identified APD measurements greater than 15 mm at any time during gestation as a crucial indicator of severe PNH and correlated with a real risk for postnatal obstructive pathology. The importance of 15 mm of APD has been affirmed by other series and is a measurement that can be used in practice to identify children with severe PNH that should certainly have prompt follow-up with a urologist soon after birth. Fig. 2 depicts the appropriate measurement of fetal renal APD and a subsequent classification scheme proposed in a recent consensus statement on PNH published by the Society for Fetal Urology (SFU). The classification system is based on renal APD measurements in the second and third trimesters.
Both prenatally and postnatally, the severity of hydronephrosis is often characterized using subjective descriptors that include terms, such as pelviectasis , caliectasis , pelvocaliectasis , mild hydronephrosis , moderate hydronephrosis , and severe hydronephrosis . A more objective method for postnatal grading of hydronephrosis was published by the SFU in 1993. The goal of the SFU 5-point classification scheme aimed at removing the subjectivity of these commonly used descriptors by replacing them with more concrete definitions. Fig. 3 depicts the 5-point grading scale and associated images for each classification. When defining hydronephrosis, measurement of the renal pelvis APD is preferable during the fetal period, and the SFU grading scale is appropriate for evaluating postnatal images.
Etiology of Hydronephrosis
The problem with identifying PNH is subsequently determining which patients harbor a substantial risk for true postnatal pathology versus patients with benign or transient hydronephrosis of fetal development that is likely to resolve. A positive correlation exists between increasing hydronephrosis grade and true postnatal pathology. Data from a meta-analysis found that the PNH defined as mild, moderate, or severe hydronephrosis (using trimester-based renal APD measurements for each classification) conferred a 12%, 45%, and 88% risk, respectively, for a subsequent diagnosis of true urinary tract pathology. The most common causes of PNH are illustrated in Table 3 .
