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Keywords:

  • congenital abnormalities;
  • sensitivity and specificity;
  • ultrasonography

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Objectives

To determine the detection rate of fetal malformations and chromosomal abnormalities and the rate of false-positive ultrasound diagnoses at routine ultrasound examinations carried out by specially trained midwives in an unselected pregnant population from 2000 to 2005, and to describe the consequences of true-positive and false-positive ultrasound diagnoses of fetal malformations.

Methods

A retrospective analysis was undertaken of all babies born in Malmö, Sweden, between January 2000 and December 2005 by mothers residing in Malmö and of all fetuses with an ultrasound diagnosis of malformation made in the same time interval at the two units performing all routine pregnancy scans in Malmö. All women underwent two routine scans, at 18 and 32 weeks, including scrutiny of the fetal anatomy. Detection rates and false-positive rates were calculated per fetus.

Results

The prevalence of chromosomally abnormal fetuses was 0.31% (52/16 775); that of chromosomally normal fetuses with major and minor malformations was 1.80% (302/16 775) and 1.32% (222/16 775), respectively. The detection rate of fetuses with major malformations but normal chromosomes was 68% (205/302), with a detection rate at < 22 weeks of 37% (112/302). In addition, 46% (24/52) of all chromosomally abnormal fetuses were diagnosed before birth because a malformation was detected at ultrasound imaging, 33% (17/52) being detected at < 22 gestational weeks. In all, 68 pregnancies were terminated because of an ultrasound diagnosis of fetal malformation (0.4% of all pregnancies and 47% of the pregnancies in which a fetal malformation was detected by ultrasound examination before 22 weeks). A false-positive ultrasound diagnosis of malformation was made in 0.19% (31/16 180) of the normally formed fetuses and in 20 (0.12%) fetuses the abnormal finding persisted during pregnancy. No fetus assigned a false-positive diagnosis was lost by termination of pregnancy, but most were subjected to one or more unnecessary interventions before birth (e.g. amniocentesis), at birth (e.g. Cesarean section) or after birth (e.g. electrocardiogram, X-ray, ultrasound examination or treatment with antibiotics).

Conclusions

In a screening program consisting of one fetal anomaly scan at 18 weeks and another at 32 weeks the detection rate of major malformations in chromosomally normal fetuses was 68% with a detection rate of 37% at < 22 weeks. The corresponding detection rates of chromosomally abnormal fetuses were 46% and 33%. Fewer than one in 500 screened fetuses had an ultrasound diagnosis of an anomaly that was not confirmed after birth. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Ultrasound examination in the second trimester of pregnancy, including scrutiny of the fetal anatomy with the aim of detecting fetal malformations, is offered routinely to many pregnant women in Western countries. It is important to provide pregnant women with correct information on the diagnostic performance of routine ultrasound examination, so that they know what they can expect from the examination before they consent to it. The diagnostic performance of routine ultrasound examinations performed in the 1980s and 1990s has been described in several publications1–21. Because of improved ultrasound technology offering better resolution, and improvement in knowledge and experience of ultrasound examiners, detection rates of fetal malformations may have increased since the 1990s. To the best of our knowledge, there is only one report on the diagnostic performance of routine ultrasound examinations performed after the year 200022.

The aims of this study were to determine the detection rate of fetuses with congenital structural abnormalities and chromosomal abnormalities, and the rate of false-positive ultrasound diagnoses, at routine ultrasound examinations carried out by specially trained midwives in an unselected pregnant population from 2000 to 2005, and to describe the consequences of true-positive and false-positive ultrasound diagnoses of fetal malformations.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

A retrospective analysis was undertaken of all babies born at the labor ward of Malmö University Hospital, Malmö, Sweden, between 1 January 2000 and 31 December 2005 by mothers residing in Malmö, and of all fetuses with an ultrasound diagnosis of malformation made in the same interval at the ultrasound department of Obstetrics and Gynecology, Malmö University Hospital or at the private Cura clinic, Malmö. All ultrasound examinations of pregnant women residing in Malmö during the study period were performed at these two ultrasound units. From the population described above we excluded babies born with a malformation diagnosed by ultrasound imaging in 1999, and pregnant women who participated in a randomized controlled trial23 and therefore underwent a 12-week routine scan instead of an 18-week routine scan.

During the study period, all pregnant women residing in Malmö were offered two routine ultrasound examinations during pregnancy, one at 18 gestational weeks and another at 32 gestational weeks. Fetuses with estimated fetal weight on the basis of ultrasound fetometry of > 5% below the mean fetal weight for gestational age24 at the routine 32-week scan underwent a second weight estimation by ultrasound imaging at 35 weeks. Additional ultrasound scans at 28 and 35 weeks were offered routinely to women with multiple pregnancies or diabetes, and all women with prolonged pregnancy were offered ultrasound examination for fetal weight estimation at 42 weeks. The uptake of these routine scans was virtually 100%. Because of the extensive routine scanning program described above indicated scans were very rare.

Chromosomal analysis was offered to women ≥35 years old, to women with risk factors for chromosomal abnormality other than advanced maternal age, and to those carrying a fetus with a structural anomaly detected at ultrasound examination. Gestational age was calculated from the fetal biparietal diameter or from the fetal biparietal diameter and femur length25.

During the study period from 1 January 2000 to 31 December 2005, nine specially trained midwives performed the routine ultrasound examinations (one midwife in the private clinic and eight in Malmö University Hospital). Their experience with routine ultrasound examination at the start of the study varied between 3 months and 21 years (median, 13 years). The examinations were performed transabdominally with high-quality ultrasound systems (ALOKA SSD 5000 or 5500, Aloka Co. Ltd., Tokyo, Japan; or Philips ATL HDI 5000, Philips ATL, Solingen, Germany) equipped with 3.5–5-MHz transabdominal transducers. The scheduled examination time was 30 min. The first routine scan performed at 15–22 weeks (18-week scan) included scrutiny of the fetal anatomy following a checklist. The following structures were examined: fetal skull and brain (midline echo, cavum septi pellucidi, ventricles, cerebellum, cisterna magna), neck, profile, face, thorax, four-chamber view of the heart, stomach, insertion of the umbilical cord, number of vessels in the umbilical cord, kidneys, bladder, spine (in three planes), arms, legs, hands and feet. Counting of the toes and fingers was not obligatory, and gender was not examined. ‘Soft markers’ were not actively sought. From the end of 2002 the routine examination of the heart included examination of the outflow tracts. The second routine scan at 30–34 weeks (32-week scan) included fetal weight estimation and a survey of the fetal anatomy as described above. Additional scans offered routinely to high-risk women (multiple pregnancy, diabetes, fetal weight deviation >5% below the mean fetal weight for gestational age, prolonged pregnancy) did not include a detailed examination of fetal anatomy. If a midwife performing a scan suspected a fetal structural anomaly, the woman was referred to an obstetrician qualified in obstetric ultrasound imaging for diagnosis and management. The ultrasound diagnosis made by the obstetrician was regarded as the prenatal diagnosis. A malformation suspected by the scanning midwife but not confirmed by the obstetrician was not considered to represent a prenatal diagnosis of malformation.

All newborns underwent routine pediatric examination after birth and were followed up with regard to congenital malformations until they were discharged from postnatal care. A system was in place whereby secretaries at the postpartum and neonatal wards forwarded information on all newborns with a malformation diagnosis to the ultrasound unit of the Department of Obstetrics and Gynecology, Malmö University Hospital. Fetuses with an ultrasound diagnosis of malformation were followed up until a final diagnosis had been made by pediatric investigation, or by autopsy or other investigations of aborted fetuses and stillborn babies. Loss to follow-up is explained by failure to trace fetuses/babies whose mothers moved out of the area or delivered in other hospitals, or failure to trace babies who underwent outpatient follow-up only.

In collaboration with pediatricians we classified congenital malformations as being clinically important (major) or clinically less important (minor). Major malformations are lethal or associated with possible survival with severe or moderate immediate or long-term morbidity. Minor malformations are associated with minor or no morbidity. The following malformations were classified as minor: ventricular septal defect of the heart (VSD), atrial septal defect of the heart (ASD), VSD plus ASD, lachrymal duct obstruction, extra spleen, balanic hypospadias, split scrotum, cleft uvula, ankyloglossia, polydactyly, syndactyly, malformation of the sternocleidomastoid muscle, tracheomalacia or laryngomalacia, nasal septal defects or deviations, and some malformations of the ear, mouth and face. Fetuses and newborns with more than one malformation but not having a multiple malformation sequence or a syndrome were assigned one main malformation diagnosis by the authors, the most serious malformation being regarded as the main diagnosis. For example, one fetus with a reduction anomaly of the arm and a diaphragmatic hernia was assigned a main diagnosis of diaphragmatic hernia.

The main malformation diagnoses were also classified as belonging to one of the following groups modified from the International Classification of Diseases 10: (1) body wall, skin, (2) respiratory system, (3) circulatory system, (4) central nervous system, (5) ear, face, neck, (6) eye, (7) gastrointestinal tract, (8) genitals, (9) kidneys and urinary tract, (10) liver, gallbladder, (11) cleft lip and palate, (12) mouth, throat, (13) musculoskeletal system, (14) multiple malformation sequence, (15) syndromes and (16) chromosomal anomalies. Babies with any of the following anomalies but no other malformations were classified as being normally formed: preauricular appendage, gill duct fistula, hip luxation, persistent ductus arteriosus in a preterm baby, hemangioma or nevus, retentio testis, inguinal hernia, umbilical hernia, suspected but not verified heart malformation (e.g. heart murmur at pediatric examination but no heart malformation confirmed). Sonographic soft markers (choroid plexus cysts, mild pyelectasis, echogenic bowel, echogenic focus of the heart) or two vessels in the umbilical cord were not considered in our analysis. An ultrasound diagnosis of ventriculomegaly was made if the width of the atria of the brain ventricles measured 10–15 mm and one of hydrocephalus was made if they measured > 15 mm.

The sensitivity (detection rate) and positive predictive value of routine ultrasound examination with regard to detection of congenital anomalies were calculated. These calculations were made per fetus, not per malformation. We present results separately for fetuses with and without chromosomal anomalies.

Definitions

A fetus with a structural malformation was considered to have been detected at a scan if the ultrasound diagnosis was in reasonable agreement with the postnatal diagnosis. For example, a fetus with hydrocephalus secondary to another brain anomaly was considered to have been diagnosed before birth if the ultrasound diagnosis was hydrocephalus, even if an exact diagnosis of the underlying disorder causing the hydrocephalus was not made until after birth. Similarly, a fetus with a syndrome was considered to have been detected if fetal anomalies were seen but the correct syndrome diagnosis was not made until after birth.

A fetus with a chromosomal anomaly was considered to have been detected only if a fetal anomaly seen at a scan resulted in fetal karyotyping showing a chromosomal anomaly. A fetus with a chromosomal anomaly was considered to have been missed if abnormal ultrasound findings did not result in fetal karyotyping, or if ultrasound findings were normal but fetal karyotyping was carried out for an indication other than a fetal anomaly, or if ultrasound findings were normal and the diagnosis was made after birth. A fetus was considered to be chromosomally normal if karyotyping before or after birth showed normal chromosomes or a balanced chromosomal anomaly, or if there were no chromosomal stigmata at pediatric examination. A fetus with an ultrasound diagnosis of an anomaly that could not be confirmed after birth or after spontaneous or induced abortion was considered to have had a false-positive ultrasound diagnosis, even if the anomaly regressed during pregnancy (e.g. regression of hydronephrosis, pericardial effusion or ventriculomegaly).

Prevalence of malformation (i.e. malformed fetuses) is defined as (number of fetuses or babies with a malformation) divided by (number of babies born plus number of malformed fetuses lost by miscarriage or termination of pregnancy after the malformation diagnosis).

Sensitivity is defined as (number of fetuses with malformations detected at ultrasound) divided by (total number of malformed fetuses and babies). Positive predictive value is defined as (number of fetuses or babies with a malformation confirmed after birth or abortion) divided by (number of fetuses with an ultrasound diagnosis of that specific malformation). We use two definitions of false-positive rate. The first is (number of normally formed fetuses with an ultrasound diagnosis of malformation) divided by (total number of normally formed fetuses/babies); this corresponds to 1 minus specificity. The second definition is (number of normally formed fetuses/babies) divided by (number of fetuses with an ultrasound diagnosis of malformation). This corresponds to 1 minus positive predictive value.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

From 1 January 2000 to 31 December 2005, 17 800 babies were born to women residing in Malmö and delivering at Malmö University Hospital. Of these, 1102 (6%) were included in a randomized trial23 and underwent their first routine scan at 12 gestational weeks instead of at 18 gestational weeks, and were excluded. The remaining 16 698 fetuses were included in our analysis. An additional 76 fetuses, which were lost as a result of pregnancy termination because of an anomaly diagnosed during pregnancy, and one fetus with an ultrasound diagnosis of gastroschisis, which was lost by miscarriage, were included in the analysis. Thus, our study population consisted of 16 775 fetuses. Among these 16 775 fetuses, 576 malformed fetuses/babies were identified, 52 with a chromosomal anomaly, 302 with major structural malformations but normal chromosomes, and 222 with minor malformations and normal chromosomes. This corresponds to a prevalence of chromosomally abnormal fetuses of 3.1 per 1000 (52/16 775), of chromosomally normal fetuses with major structural malformations of 18.0 per 1000 (302/16 775) and of chromosomally normal fetuses with minor structural malformations of 13.2 per 1000 (222/16 775). Major malformations were most common in the kidneys and urinary tract (5.5 per 1000 fetuses) and in the circulatory and musculoskeletal systems (2.2 and 2.3 per 1000 fetuses, respectively). Minor malformations were most common in the genitals, 3.8 per 1000 fetuses having a minor genital malformation, the most common of these being balanic hypospadias. Among 283 chromosomally normal fetuses with a main diagnosis of major malformation but not a multiple malformation sequence or a syndrome, eight (2.8%) had more than one malformation and were assigned one main malformation diagnosis by the authors. Among 222 fetuses with a main diagnosis of minor malformation, three (1.4%) had more than one minor malformation. The prevalence of major and minor malformations and of chromosomal anomalies is shown in Table 1. Figure 1 is a schematic drawing summarizing the ultrasound findings and outcome of the 16 775 fetuses included. Details are given below.

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Figure 1. Schematic drawing illustrating the retrospective analysis with regard to ultrasound findings and the consequences of ultrasound findings of 16 775 fetuses that underwent routine ultrasound examination. AC, diagnosis by amniocentesis at 15–17 gestational weeks for an indication other than fetal malformation detected at ultrasound examination; TOP, termination of pregnancy; US, ultrasound; US < 22 weeks, ultrasound diagnosis of malformation at < 22 gestational weeks; US ≥ 22 weeks, ultrasound diagnosis of malformation at ≥ 22 gestational weeks.

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Table 1. Prevalence of fetuses with malformations
Anomaly/malformationPrevalence per 1000 (n)
  • *

    One fetus/baby had more than one malformation.

  • Two fetuses/babies had more than one malformation.

  • Three fetuses/babies had more than one malformation.

  • ASD, atrial septal defect; VSD, ventricular septal defect.

Chromosomal anomaly3.1 (52)
Major malformation, normal chromosomes18.0 (302)
 Kidneys, urinary tract*5.5 (93)
 Circulation2.2 (37)
 Muscles, skeleton*2.3 (38)
 Central nervous system1.9 (32)
 Facial cleft*1.8 (31)
 Body wall, skin1.1 (18)
 Syndrome, multiple malformation sequence, body stalk1.1 (19)
 Gastrointestinal tract0.8 (13)
 Genitals0.7 (12)
 Respiratory system*0.2 (4)
 Eye, ear, face, neck0.3 (5)
Minor malformation, normal chromosomes13.2 (222)
 Circulation (ASD, VSD or both)2.9 (48)
 Muscles, skeleton (accessory or missing finger/toe, malformations of sternocleidomastoid muscle and others)2.9 (48)
 Cleft uvula0.1 (2)
 Genitals (balanic hypospadias or cleft scrotum)3.8 (63)
 Eye (lachrymal duct obstruction)0.06 (1)
 Respiratory system (tracheomalacia or laryngomalacia, nasal septal defects or deviations)0.5 (8)
 Ear (unspecified ear malformations)2.4 (41)
 Mouth, throat (ankyloglossia and others)0.6 (10)
 Liver, gallbladder, spleen (extra spleen)0.06 (1)

Detection rate

Detection rates for chromosomally normal fetuses with major structural malformations are shown in Table S1. The total detection rate of fetuses with major structural anomalies was 68% (205/302). Detection rates were highest for major malformations in the central nervous system (97%, 31/32), and kidneys and urinary tract (97%, 90/93). They were lowest for major malformations of the eye or ear (none detected), musculoskeletal system (32%, 12/38) and gastrointestinal tract (29%, 4/13). Most of the major structural anomalies were detected at the 18-week scan or 32-week scan: 37% (112/302) were detected before 22 weeks, 25% (75/302) at 30–34 weeks, 6% (18/302) at other ultrasound scans, and 32% (97/302) after birth.

Detection rates of chromosomally abnormal fetuses are presented in Table S2. For chromosomally abnormal fetuses the detection rate at ultrasound examination was 46% (24/52). In all, 68% (15/22) of fetuses with trisomy 18, trisomy 13, triploidy or ‘X0’ vs. 33% (8/24) of those with trisomy 21 were detected before birth because they manifested sonographic anomalies resulting in fetal karyotyping. Another six (13%) of the 46 fetuses with trisomy 21, 18, 13, triploidy or ‘X0’ were diagnosed in utero because fetal karyotyping was carried out for indications other than a fetal anomaly seen at ultrasound examination, whereas 17 (37%) were born undiagnosed. The most common ultrasound findings associated with trisomy 21 were heart malformations, thick nuchal fold, hygroma and an abnormally shaped skull. Those associated with trisomy 18 were hygroma, hydrops, and malformations of the central nervous system and heart, and those associated with ‘X0’ were hygroma and hydrops. In one case of trisomy 18, bilateral choroid plexus cysts were seen at the 18-week scan, and the obstetrician detected an enlarged ductus arteriosus but considered it to be within the normal limits; amniocentesis was not carried out until 36 weeks on the indication of intrauterine growth restriction and we considered this case as having been missed at ultrasound examination, because no malformation was suspected by the obstetrician. One fetus with triploidy was diagnosed with hydrocephalus at the routine scan, but the mother refused amniocentesis and the fetus died at 35 weeks, the final diagnosis being made only after birth; this case, too, was considered to have been missed at ultrasound examination.

The detection rate for fetuses with a main diagnosis of minor malformation was 1.4% (3/222). Forty-eight babies had an isolated VSD, ASD or both a VSD and ASD. Two babies with an isolated VSD were diagnosed in utero, one at 17 weeks and the other at 32 weeks (both VSDs were large). The only other fetus diagnosed with a minor malformation at ultrasound examination had bilateral accessory little fingers detected at 18 weeks. This corresponds to a detection rate of accessory toes/fingers of 5% (1/21).

False-positive ultrasound diagnoses of malformation in chromosomally normal fetuses

Of 258 fetuses with an ultrasound diagnosis of structural malformation, 208 (81%) had the ultrasound diagnosis confirmed, 19 (7%) fetuses (but not their mothers) were lost to follow-up, and 31 (12%) were regarded as having had a false-positive ultrasound diagnosis (Table S3). In 11/31 (35%) fetuses with a false-positive diagnosis, the ultrasound findings regressed during pregnancy (four cases of hydronephrosis, two of ovarian cyst, one of hygroma, two of ventriculomegaly, one of pericardial effusion and one of ascites). The ultrasound findings that persisted during pregnancy without a corresponding diagnosis being confirmed after birth/termination of pregnancy were: five cases of hydronephrosis, two of heart aneurysm, two cases where the fetus had abnormally short long bones and was suspected to have osteochondrodysplasia, one fetus with an abnormal shape of the skull suspected to have craniosynostosis, six cases of ventriculomegaly (10–15 mm), one case of cleft lip, one case of omphalocele (the baby had an umbilical hernia but no omphalocele), one of esophageal atresia, and one with an atrial septal defect.

Outcome for fetuses with structural anomalies detected at ultrasound examination but with normal or unknown chromosomes

All fetuses with an ultrasound diagnosis of structural malformation made at ≥ 22 weeks were born alive. Of 128 fetuses in which a malformation was diagnosed by ultrasound imaging at < 22 gestational weeks, 51 (40%) were lost by termination of pregnancy, 76 (59%) were born alive, and one fetus with gastroschisis was lost by miscarriage (1%). In 50 of the 51 terminated pregnancies the ultrasound malformation diagnosis was confirmed at autopsy: anencephaly (n = 11), myelomeningocele (n = 6), hydrocephalus (n = 6), complex heart malformation (n = 7), multiple malformation sequence (n = 3), osteochondrodysplasia (n = 4), arthrogryposis (n = 2), osteogenesis imperfecta (n = 1), reduction anomaly (n = 1), bilateral renal agenesis (n = 1), megacystis/urethral valves (n = 5), diaphragmatic hernia (n = 1), hydrops (n = 1), and cleft lip and palate (n = 1). In one case in which the pregnancy was terminated because of a reduction anomaly of the arm, autopsy also revealed a diaphragmatic hernia that had not been diagnosed during pregnancy.

Of the 31 fetuses with a false-positive ultrasound diagnosis of structural malformation—30 major malformations and one minor malformation (atrial septal defect)—all were born alive. Of the 31 fetuses born with a false-positive ultrasound diagnosis of major malformation, 20 had an ultrasound diagnosis that persisted during pregnancy, whereas 11 had ultrasound findings that regressed during pregnancy. Of the 20 fetuses with persisting major ultrasound findings, eight were karyotyped during pregnancy, two were delivered by Cesarean section because of the false-positive malformation diagnosis (one case of suspected heterozygous achondroplasia and another of suspected left heart ventricle aneurysm), one was referred for delivery in a tertiary center (suspected omphalocele that turned out to be an umbilical hernia), two newborns with a false-positive diagnosis of hydronephrosis received treatment with prophylactic antibiotics, nine babies underwent additional investigations after birth (ultrasound, computed tomography, electroencephalography, electrocardiography, etc.), whereas eight fetuses were subjected to additional follow-up ultrasound examinations during pregnancy but to no other medical interventions indicated by the malformation diagnosis either before or after birth.

Of the 11 fetuses with a sonographic anomaly that regressed during pregnancy, five underwent follow-up investigations after birth (ultrasound imaging, X-ray, electrocardiogram), and one of these five babies, who had transient hydronephrosis during pregnancy, also received treatment with prophylactic antibiotics. The remaining six fetuses with a transient anomaly had no extra examinations or treatment after birth, possibly because the pediatrician paid no attention to the prenatal diagnosis. None of these 11 fetuses was karyotyped during pregnancy.

Outcome for fetuses with abnormal karyotype

All 17 pregnancies with a chromosomal anomaly diagnosed by fetal karyotyping before 22 weeks because of a structural anomaly seen at a scan were terminated. In all, 25/52 (48%) pregnancies with a chromosomally abnormal fetus ended in pregnancy termination.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

We have shown that within the frame of our routine obstetric ultrasound program 68% of all chromosomally normal fetuses with a major malformation were detected before birth, and that 37% were diagnosed before 22 gestational weeks. In addition, 46% of all chromosomally abnormal fetuses were diagnosed before birth because a malformation was detected at ultrasound examination, with 33% diagnosed before 22 gestational weeks. In all, 0.4% of all pregnancies were terminated because of an ultrasound diagnosis of fetal malformation. A false-positive ultrasound diagnosis of malformation was made in 0.19% of the normally formed fetuses, but in only 0.12% did the abnormal ultrasound finding persist during pregnancy. No fetus assigned a false-positive diagnosis of malformation was lost by termination of pregnancy, but most were subjected to one or more unnecessary interventions before birth (e.g. amniocentesis), at birth (e.g. Cesarean section) or after birth (e.g. electrocardiogram, X-ray, ultrasound examination or treatment with antibiotics).

The strength of our paper is our careful follow-up. The prevalence of major malformations in our study population was similar to or higher than that reported in the Swedish Registry of Congenital Malformations 1999–2004 (Swedish Registry of Congenital Malformations. Stockholm, Sweden: The National Board of Health and Welfare, Centre for Epidemiology, http://www.socialstyrelsen.se/). This indicates that most malformed fetuses were identified, and that our detection rates are unlikely to have been overestimated. Another strength is our reporting of false-positive ultrasound diagnoses and of the consequences of both true-positive and false-positive ultrasound findings. In many publications on the diagnostic performance of routine ultrasound examination in pregnancy, false-positive ultrasound diagnoses of malformations are much less well described than detection rates and/or their consequences are not commented on5–8, 10–16, 18–21. A weakness of our paper is that it is retrospective. Another limitation is our classification of cardiac malformations. We classified all VSDs, ASDs and VSDs plus ASDs as minor malformations. The reason is that we were unable to trace all babies with these types of malformation after they were discharged from the postpartum ward in good condition. Most VSDs and ASDs were small, but we cannot exclude the possibility that some of them turned out to be clinically significant at outpatient pediatric follow-up. Therefore, our detection rate of what others might have classified as major heart malformations is likely to have been slightly overestimated.

It is almost meaningless to compare detection rates between studies, because the variation in detection rate reflects not only differences in examination skill and quality of equipment but also differences in the definition of agreement between prenatal and postnatal diagnosis, and differences in definitions of malformation, detection rate and quality of follow-up. The problem of defining agreement between prenatal and postnatal diagnoses has been discussed by Boyd et al.4. Our detection rate of major malformations at < 22 weeks (37%) falls between the lowest (22%) and highest (80%) reported in studies where data were collected after 19901–3, 22. In most studies, as well as in ours, detection rates were highest for malformations in the central nervous system and kidneys/urinary tract, and lowest for malformations of the skeleton and circulatory system1, 9, 26. Our detection rate of isolated VSDs and ASDs was very low. However, our detection rate of complex heart malformations (70%, 21/30) was higher than our detection rate in 1990–1996 (32%, 8/25), when a similar audit of 21 477 pregnancies was performed (Leite and Valentin, unpubl. data). We believe that our increase in detection rate of complex heart malformations is an effect not only of improved resolution of modern ultrasound equipment but also of intensified training and education of our ultrasound examiners. We undertook an educational effort to improve the detection rate of heart malformations in 2002.

In some publications, false-positive ultrasound diagnoses of fetal malformations are fairly well1, 2, 22 or very well described and discussed4, 9, 26, 27. Reported false-positive rates with regard to fetal malformations (defined as 1—specificity) are usually < 0.1%1, 4, 9, 12, 20, 21, 26, but in two studies the false-positive rate was as high as 0.48% and 0.40%14, 22. The variation in reported false-positive rates may be explained at least partly by differences in definitions, e.g. whether or not ultrasound findings that were subsequently refuted or that regressed during pregnancy (such as dilated kidney pelvis) were included. In our study as well as in others1, 9, 26, 27 hydronephrosis was a common false-positive diagnosis. False-positive diagnoses of hygroma, ascites, intra-abdominal cysts, heart malformation4, esophageal atresia27, abnormally shaped head and cleft lip9, 26 have also been reported by others. In our study, all prenatal diagnoses of clubfoot were correct, whereas false-positive diagnoses of clubfoot were described in several studies4, 9, 26, 27. It is important to bear in mind that some diagnoses may have been incorrectly classified as false positive. For example, fetuses with mild ventriculomegaly may be discharged from hospital with a diagnosis of ‘normal newborn’, but a substantial proportion of them develop psychomotor problems later in life28. Another reason for incorrectly classifying ultrasound diagnoses as false is incomplete postnatal investigation.

Even though our work may be regarded as an audit of our own routine obstetric ultrasound activities, we believe that our results may be of general interest, because they show what one can expect today from routine obstetric ultrasound examinations carried out in an unselected pregnant population, when one of the aims of the examination is to detect fetal malformations. A routine ultrasound scan with normal findings is a very positive experience for parents-to-be29. There is some evidence, albeit weak (there are no randomized trials), that prenatal diagnosis may improve the prognosis for children with certain malformations of the circulatory system30–35, gastroschisis, duodenal obstruction and esophageal atresia, at least if intensified surveillance of fetuses with the latter diagnoses were to be instituted36–38. However, false-negative diagnoses have negative long-term effects on the mental health of mothers39, and they result in bitterness and skepticism towards healthcare in general39. False-positive diagnoses may at best result only in unnecessary parental worry but at worst in pregnancy termination being undertaken on incorrect grounds40, 41. It is important that parents-to-be receive high-quality information on prenatal diagnosis, so that they know what they can expect from it29. To maximize the good and minimize the harm of routine ultrasound examination in pregnancy, it is also important that both those who perform the routine scans and those who are responsible for making the final prenatal diagnosis are well educated, trained and skilled.

SUPPORTING INFORMATION ON THE INTERNET

The following supporting information may be found in the online version of this article:

Table S1 Detection rate for major malformations in chromosomally normal fetuses.

Table S2 Detection rate for chromosomal anomalies.

Table S3 Positive predictive value and false-positive rates (1—positive predictive value) of ultrasound diagnoses in chromosomally normal fetuses.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Dr Sissel Saltvedt retrieved the detailed information on the 1102 fetuses included in the randomized controlled trial that were excluded from this study.

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information
FilenameFormatSizeDescription
TableS1.doc79KSupporting Information: Table S1 Detection rate for major malformations in chromosomally normal fetuses.
TableS2.doc33KSupporting Information: Table S2 Detection rate for chromosomal anomalies.
TableS3.doc80KSupporting Information: Table S3 Positive predictive value and false-positive rates (1—positive predictive value) of ultrasound diagnoses in chromosomally normal fetuses.

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