Key Points
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The main objective of routine midtrimester ultrasonography (US) is to provide accurate diagnostic information for the delivery of optimised antenatal care, including delivery planning.
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Routine early US is beneficial because of better estimates of gestational age.
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Routine US examination leads to earlier detection of clinically unsuspected fetal malformations and earlier detection of multiple pregnancies.
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Individuals who routinely perform obstetrics scans should have specialised training for diagnosis during pregnancy.
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The Eurofetus trial likely approaches the most accurate sensitivity rates for US in the detection of anomalies (∼50%–70%).
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If one screening US examination is performed, the optimal timing is at 18 to 22 weeks of gestation, allowing for good visualisation of anatomy and is early enough to allow completion of prenatal diagnostic procedures with options for termination if desired.
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Routine use of US screening in the third trimester is currently not supported by available data; however, US that guides delivery planning is encouraged.
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US-suspected structural malformations as well as hydrops fetalis and congenital neuromuscular disorders, among others, should be considered for delivery at a tertiary care centre with availability of neonatologists and paediatric subspecialties.
Most congenital anomalies and adverse pregnancy outcomes occur in pregnancies without known risk factors. That 75% of congenital malformations are found in low-risk populations and 90% of infants born with congenital anomalies are born to women without risk factors suggest that routine performance of second and third trimester ultrasound (US) screening in all pregnancies is beneficial. The prenatal diagnosis of an anomaly will then assist in the optimisation of antenatal care resulting in the best possible outcomes for the mother and fetus and optimise delivery planning, which includes timing and location. Although performed routinely in many centres, the extent of benefit of such prenatal US screening on pregnancy and neonatal outcomes remains unproven.
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) has published a practice guideline for the performance of routine midtrimester fetal US and states that routine US between 18 and 22 weeks aims to provide accurate diagnostic information to provide the best outcome for the mother and the fetus. The recommended components of a standard prenatal US examination, are described in Box 20.1 . The ISUOG suggests that midtrimester US between 18 and 22 weeks of gestation represents a period that allows timely detection of anomalies and the ability to accurately date a pregnancy (more accurate the earlier it is done) ( Fig. 20.1 ).
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Presence or absence of fetal cardiac activity, fetal heart rate and rhythm
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Fetal number
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Fetal presentation
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Assessment of amniotic fluid volume
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Placental appearance and location (see Figs. 20.3 and 20. 4 )
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Umbilical cord vessel number and placental insertion site, if technically possible
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Fetal biometry (biparietal diameter, head circumference, femoral length, abdominal circumference) (see Fig. 20.8 )
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Fetal anatomic survey (18–20 weeks)
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Includes the following assessments: head (intact cranium, cavum septi pellucidi, midline falx, thalami, cerebral ventricles, cerebellum, cisterna magna, choroid plexus), face (orbits, mouth, upper lip intact), neck (absence of masses), chest and heart (shape and size of chest and lungs, cardiac activity present, four-chamber view of heart, aortic and pulmonary outflow tracts, diaphragmatic hernia), abdomen (stomach in normal position, bowel not dilated, both kidneys present, cord insertion site, bladder), skeleton (spine, masses, arms and hands, legs and feet) and genitalia
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Presence or absence of fetal movement
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Evaluation of each fetus of a multiple gestation
In the United States, the American College of Obstetricians and Gynecologists (ACOG) supports the use of US to identify a congenital structural anomaly, to evaluate for an abnormality in fetal growth or when there is a medical indication. The ACOG advises against the nonmedical use of prenatal US. In 2014, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) hosted a workshop to address indications, frequency, yield and cost effectiveness for US during pregnancy. The NICHD published a summary of the consensus among the NICHD, ACOG, Society for Maternal-Fetal Medicine (SMFM), American Institute of Ultrasound in Medicine (AIUM), American College of Radiology, Society for Pediatric Radiology and Society of Radiologists in Ultrasound.
The workshop consensus agreed upon the following benefits of US during pregnancy:
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US provides an accurate determination of gestational age, fetal number, cardiac activity, placental location and diagnosis of major fetal anomalies. (Examples of these are seen in Figs. 20.2–20.8 .)
• Fig. 20.1
A, Ventriculomegaly diagnosed by 19-week ultrasound. B, Choroid plexus cyst diagnosed by 20-week ultrasound. Caliper demarcates the cyst, and arrow indicates the choroid plexus. C, Anencephaly diagnosed at 15 weeks.
• Fig. 20.2
Twenty-three-week ultrasound image demonstrating nasal bone (left arrow) and corpus callosum with pericallosal artery (right arrow).
• Fig. 20.3
A–C, Placenta previa with a morbidly adherent placenta. D–F, Vasa previa.
• Fig. 20.4
A, Chorioangioma seen on 36-week ultrasound. B, Colour Doppler demonstrating increased vascularity of chorioangioma.
• Fig. 20.5
Colour Doppler demonstrating ventricular septal defect diagnosed on 21-week anatomy ultrasound in apical ( A ) and axial ( B ) views.
• Fig. 20.6
Fetal pelvic mass in female fetus noted at 29-week ( A ) and 33-week ( B ) ultrasound images.
• Fig. 20.7
Fetal hydrops seen on 23-week ultrasound. A, Scalp oedema. B, Abdominal ascites with skin oedema. C, Pleural effusion. D, Abdominal ascites. E, Sagittal section of abdominal ascites.
• Fig. 20.8
Standard fetal biometry. Fetal weight can be estimated by obtaining measurements such as biparietal diameter (BPD), head circumference (HC), femoral diaphysis length (FL) and abdominal circumference (AC).
Results from prediction models can be compared with fetal 8th percentiles from nomograms.
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US is safe for fetuses when used appropriately.
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US improves the detection of fetal growth abnormalities and changes in amniotic fluid volume.
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In the absence of an indicated specific first-trimester examination, the optimal timing for a single US examination is at 18 to 20 weeks of gestation.
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The benefits and limitations of US should be discussed with all patients.
Ultrasonography for the Detection of Congenital Anomalies
Although it is both recommended and routinely performed, the sensitivity of routine US screening to detect fetal anomalies in an unselected population remains controversial. Large systematic reviews report a 16% to 44% detection of anomalies when completed before 24 weeks of gestation, with higher detection rates of major and lethal anomalies (see Fig. 20.1 to 20.7 ). The overall detection rate for lethal anomalies is as high as 84%, although the sensitivity of detection varies by anomaly and factors such as gestational age, maternal body mass index and operator skill and experience. The consensus of these reviews agrees that at least a single US should be routinely offered to all pregnant women between 18 weeks and 22 weeks of gestation to allow for gestational dating, evaluation of fetal anatomy, diagnosis of multiple gestation and chorionicity, abnormal placentation and an assessment of the cervix.
In a 2015 Cochrane review of trials of routine US before the 24th week of pregnancy, routine early pregnancy US significantly increased the detection of fetal abnormalities (relative risk (RR), 3.46; 95% confidence interval (CI), 1.67–7.14). However, only two trials included in this review evaluated the ability to detect fetal structural anomalies, the Helsinki trial and the Routine Antenatal Diagnostic Imaging with Ultrasound (RADIUS) trial. Notably, a number of factors that affect the detection rate were not included in determination of the RR. These include gestational age, type of malformation, number of US examinations performed by the operator, operator experience and the prevalence of the anomaly in the population. In addition, US imaging technology has improved significantly since these trials. Next we review the trials of routine second trimester US while appreciating that recent trials are lacking (see Figs. 20.1, 20.2, and 20.5 to 20.7 ).
Helsinki Trial
The Helsinki trial was performed between 1986 and 1987 and randomly assigned 4691 women to routine screening US at 16 to 20 weeks and compared outcomes with 4619 control participants who only underwent clinically indicated US. Ninety-five percent of all pregnant women in Helsinki participated in the study during the study period. Seventy-seven percent of women in the control group underwent US examination during pregnancy. Routine second trimester US screening increased detection of fetal anomalies. Approximately 50% of serious anomalies were detected; 36% in the City Hospital and 77% at the University Hospital. There was a significant reduction in the perinatal mortality rate (4.6 per 1000 vs 9.0 per 1000) in the screened population, most likely related to the termination of anomalous fetuses resulting in fewer fetal and neonatal deaths due to congenital anomalies.
RADIUS Trial
The RADIUS Trial was a multicentre study conducted in the United States between 1987 and 1991. The study randomised 15,151 women to either screening US examinations at both 15 to 22 weeks and 31 to 35 weeks or to US for obstetric indications only. Forty-five percent of the control group had at least one US examination. Routine second trimester US screening resulted in an increased detection of anomalies (34.8% vs 11%). Of the anomalies, half were detected before 24 weeks of gestation in the screened group. However, despite the US detection of anomalies, there was no improvement in perinatal outcomes with early detection. There was also no difference in the number of abortions performed for fetal anomalies, and there was no improvement in survival among anomalous fetuses. Antenatal detection of fetal anomalies did not improve survival over that with postnatal diagnosis. Again, this trial was conducted at a time with a more limited US resolution than used today.
Eurofetus Study
The Eurofetus Study was the largest study of routine US in unselected pregnancies. This study was conducted between 1990 and 1993. Women were routinely scanned between 18 to 22 weeks’ gestation in 61 European obstetric units, and data were prospectively collected. The sensitivity for the detection of all anomalies was 56.2%. The detection rate was higher for major (73.7%) versus minor (45.7%) anomalies and higher for anomalies of the central nervous system (CNS) (88.3%) and the urinary tract (88.5%) than for cardiac abnormalities (20.8%–38.8%). Overall, 44% of anomalies and 55% of severe anomalies were detected before 24 weeks. Cardiac and facial defects were diagnosed later in pregnancy than abnormalities of the CNS or urinary tract.
Several factors should be considered when reviewing these trials. Notably, one must consider the prevalence of anomalies, population demographics and study design. Detection rates for anomalies are also contingent on follow up. Anomaly prevalence rates reported in the literature range from 0.3% to 3.2%. Studies with low prevalence rates possibly represent incomplete follow up.
Clinical characteristics of the population studied also affect the detection rate and the generalizability of the outcomes and data. For instance, in the RADIUS trial, only 28% of the eligible women were included in the study after exclusion parameters were applied. In particular, the study design did not include women who wished to undergo US and those who might consider termination in the event of a fetal anomaly. This significantly alters the study population and decreases the generalizability. The Helsinki trial was more representative in that it included 95% of pregnant women in the defined cohort and thus more reliably described the consequences of detecting fetal anomalies.
The primary study endpoints of the RADIUS trial were the neonatal consequences of prematurity, not fetal anomalies. As such, the RADIUS trial was not powered to detect differences associated with anomalies, including neonatal complications and financial implications. For example, patients with fetal congenital heart disease, specifically single ventricles and truncal abnormalities, have better outcomes when the diagnosis is made prenatally rather than postnatally.
The location of the US examination matters. Both the RADIUS trial and the Helsinki trial demonstrated that examinations performed in a hospital or tertiary care setting identified a higher proportion of existing anomalies than those performed in community centres. This is likely secondary to sonographer or operator experience and possibly differences in equipment rather than the location per se. Furthermore, both the RADIUS and Helsinki trials were performed in the 1980s when US technology was still new and thus may not be generalizable to current practice with advanced US technology more readily available throughout the world.
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