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Obstetrics & Gynecology 1999;93:382-386
© 1999 by The American College of Obstetricians and Gynecologists
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ORIGINAL RESEARCH

Objective Diagnosis of Micrognathia in the Fetus: The Jaw Index

DARIO PALADINI, MD, TIZIANA MORRA, MD, ADELE TEODORO, MD, AGATA LAMBERTI, MD, FORTUNATO TREMOLATERRA, MD and PASQUALE MARTINELLI, MD

From the Departments of Obstetrics and Gynecology and Pathology, University Federico II of Naples, Naples, Italy.

Address reprint requests to: Dario Paladini, MD, Via Cimarosa, 69, 80127- Naples, Italy, E-mail: paladini{at}cds.unina.it


   Abstract
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Abstract
Materials and Methods
 Results
 Discussion
 References
 
Objective: To provide an objective and accurate tool to diagnose micrognathia in the fetus.

Methods: The anteroposterior and laterolateral diameter of the mandible were measured in 262 normal fetuses between 12 and 37 weeks’ gestation and plotted against gestational age and biparietal diameter (BPD). The jaw index (anteroposterior mandibular diameter/BPD x 100) was then tested against the usual subjective method for diagnosing micrognathia, consisting of the evaluation of the facial profile, in a population of 198 malformed fetuses, 11 of which had micrognathia at necropsy or birth.

Results: The mandibular growth was linearly correlated with gestational age and BPD. Using a cutoff level of less than 23, the jaw index had a 100% sensitivity and 98.1% specificity in diagnosing micrognathia, in comparison with 72.7% and 99.2% shown by the subjective evaluation of the fetal profile. With a cutoff of 21, it yielded a positive predictive value of 100%.

Conclusion: We demonstrated the linear relationship between mandibular growth and gestational age or BPD. In addition, we validated the jaw index as an objective tool for diagnosis of micrognathia in the fetus.

Facial congenital anomalies are frequently associated with chromosomal aberrations and genetic syndromes.1,2 In this group of malformations, there are many anomalies the diagnosis of which relies almost entirely on the subjective evaluation of the facial profile; micrognathia is one of these, although recently the growth of the fetal mandible throughout pregnancy has been evaluated and normative data have been provided.3,4 Micrognathia is associated with chromosomal abnormalities in 66% of cases,5 and a variable degree of micrognathia has been reported in over 80% of cases of trisomy 18 and triploidy at autopsy.1,2 Several attempts have been made to identify objective criteria to diagnose micrognathia after birth by x-ray or computed tomographic scan.6,7

The aims of this study were to construct a growth chart of the mandible throughout pregnancy, to identify an objective criterion by which to diagnose micrognathia in utero, and to validate it in 11 cases of micrognathia detected in utero and confirmed at necropsy or after birth.


   Materials and Methods
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Abstract
 Materials and Methods
Results
 Discussion
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All pregnant women between 12 and 37 weeks’ gestation consecutively referred to our institution from January 1996 through March 1996 for routine second- and third-trimester scan were evaluated for possible enrollment in the study group. The design of the study was cross-sectional and therefore each patient was scanned only once. Entry criteria included singleton pregnancy; gestational age confirmed by an early second-trimester biparietal diameter (BPD) measurement; fetal abdominal circumference greater than the 10th percentile for gestational age, to exclude the presence of fetal growth restriction (FGR); absence of congenital anomalies; normal uteroplacental Doppler velocimetry, to exclude patients at risk of developing FGR; and the absence of maternal diseases possibly affecting fetal growth (diabetes, pregnancy-induced hypertension, and other systemic diseases). Of the initial 314 women, 52 (16.6%) were excluded, leaving 262 cases in the study group. Exclusion criteria included persistent unfavorable fetal lie in 14 (4.5%), pregnancy dating discordant with last menstrual period in 10 (3.2%), fetal malformations in nine (2.9%), abnormal uteroplacental Doppler velocimetry in five (1.6%), maternal diseases in five (1.6%), FGR in five (1.6%), and twinning in four (1.3%). Ultrasound examinations were performed by two authors (DP and TM) using TOSHIBA 270A and TOSHIBA Powervision ultrasound systems (TOSHIBA Corp., Tochigi-Ken, Japan) with 3.75 MHz and 3.75–6.0 MHz multifrequency convex probes, respectively.

The size of the fetal mandible was assessed on an axial plane at the base of the cranium just caudad to the lower dental arch, where the whole horseshoe mandible is imaged. Anteroposterior and laterolateral diameters were measured as follows: the laterolateral diameter was traced joining the bases of the two rami; the anteroposterior diameter from the symphysis mentis to the middle of the laterolateral diameter (Figure 1Go). Care was taken to achieve the correct plane and to avoid inadvertent partial inclusion of the rami within the calipers. Mandibular diameters were plotted against gestational age and BPD as independent variables to build the growth charts. The jaw index was then calculated as follows: anteroposterior mandibular diameter/BPD x 100. In the first 20 fetuses, intraobserver and interobserver variability were assessed by calculating the Cronbach alpha reliability coefficient for two measurements taken from the same operator (DP) and from the two operators (DP and TM) on different frames. In particular, the first operator (DP) measured the mandibular diameters as described above and noted the values calculated on two different frames (intraobserver variability); then, the second operator (TM) repeated the scan and remeasured the diameters on a newly acquired frame, not knowing the results of the measurements from the former operator. The interobserver variability was assessed by comparing the first set of measurements by the first operator with the set of measurements taken by the second operator. Twelve fetuses were scanned by DP first, eight by TM first. Regardless of who scanned the patient first, intraobserver variability was evaluated on repeated measurements taken by DP only. The Cronbach alpha reliability coefficients were 0.97 for intraobserver variability and 0.90 for interobserver variability, indicating a good repeatability of the measurement. Statistical analysis was performed with the statistical package SPSS 7.5 for Windows 95 (SPSS Inc, Chicago, IL). The data gathered from the 262 normal fetuses were submitted to regression analysis using gestational age and BPD as independent variables. The best-fit curve (among linear, quadratic, cubic, and exponential) was selected to express the relationship between mandibular diameters and BPD or gestational age. Confidence limits were calculated for all variables. Percentiles were developed for the jaw index. The Mann-Whitney U test was used to compare jaw index values between normal fetuses and fetuses with micrognathia. P < .05 was considered statistically significant.



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  		Figure 1. Transverse scan of the fetal mandible. A to A = laterolateral diameter; B to C = anteroposterior diameter.

 
Once the reference curves and the jaw index had been developed, they were prospectively tested against a population of 210 fetuses referred to our unit between April 1996 and December 1997 for confirmation and treatment of congenital anomalies. No confirmation of the prenatal diagnosis could be gathered in 12 cases, which were excluded from the study, leaving 198 cases for analysis, 11 of which had micrognathia. In these fetuses, the diagnosis of micrognathia was first subjectively hypothesized on the basis of the evaluation of the facial profile and then validated using mandibular growth charts and the jaw index. On the basis of the data from the normal fetuses, a cutoff value of 24, representing the fifth percentile, was prospectively chosen to test the diagnostic accuracy of this variable. Autopsy reports and pediatric charts were used for confirmation. Armed Force Institute of Pathology criteria were used to diagnose micrognathia in postmortem examinations.8 Maternal mean age ± standard deviation was 29 ± 5 years in the group of 262 normal fetuses and 31 ± 0.6 years in the group of malformed fetuses. Mean parity was 0.8 ± 0.8 and 0.9 ± 0.9, respectively. All women were white.


   Results
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Abstract
 Materials and Methods
 Results
Discussion
 References
 
The best equation describing the relationships between mandibular diameters and gestational age or BPD in the population of 262 normal fetuses was the linear one and, for both diameters, the correlation with BPD was stronger than the correlation with gestational age, as evident from the lower regression B coefficients in Table 1Go. The reference curves for mandibular diameters throughout pregnancy, obtained from the same population, are shown in Figures 2Go and 3Go. On these curves, mandibular measurements of the fetuses with confirmed micrognathia are overlayed. The data for these 11 fetuses with micrognathia, including gestational age at diagnosis, associated malformations, karyotype, and pregnancy outcome, are given in Table 2Go. As evident from Figures 2Go and 3Go, micrognathia affects the growth in the sagittal plane more than that in the coronal plane. Data regarding the jaw index are shown in Figure 4Go and Table 3Go. This index allows a clear separation between micrognathic and normal fetuses. Only one fetus with micrognathia had a jaw index greater than 21, and only two fetuses without micrognathia had a jaw index less than 22. The diagnostic accuracy of this objective method with different cutoff levels is compared with subjective evaluation of the fetal facial profile in Table 4Go. Using the latter method, two false-positive and three false-negative diagnoses of micrognathia were recorded. Among the false-negative diagnoses, one case is included in which the fetal profile could not be evaluated because of the presence of both an extremely unfavorable fetal lie and oligohydramnios. In this case, the diagnosis of micrognathia was based only on the measurement of the jaw index and was confirmed after birth.


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  		Table 1. Linear Regression Analysis of Mandibular Diameter Compared With Biparietal Diameter and Gestational Age as Independent Variables
 


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  		Figure 2. Growth chart for the mandibular anteroposterior diameter (mean and 95% confidence limits) plotted against gestational age (wk) and biparietal diameter (mm) (crosses = normal fetuses; circles = fetuses with micrognathia).

 


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  		Figure 3. Growth chart for the mandibular laterolateral diameter (mean and 95% confidence limits) plotted against gestational age (wk) and biparietal diameter (mm) (crosses = normal fetuses; circles = fetuses with micrognathia).

 

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  		Table 2. Characteristics of the 11 Fetuses With Micrognathia Confirmed at Necropsy or After Birth
 


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  		Figure 4. Boxplot of the jaw index in normal fetuses and fetuses with micrognathia. Boxes indicate quartiles (25th and 75th percentiles), and vertical bars indicate ranges.

 

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  		Table 3. The Jaw Index: Statistics and Percentiles in the Normal Population of 262 Fetuses
 

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  		Table 4. Diagnostic Accuracy of the Jaw Index and Evaluation of the Facial Profile in the Diagnosis of Micrognathia in the Population of 198 Malformed Fetuses
 

   Discussion
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Abstract
 Materials and Methods
 Results
 Discussion
References
 
The growth process of the fetal mandible is impaired in three groups of diseases: chromosomal aberrations, skeletal dysplasias, and primary mandibular disorders. The last group of diseases includes the Robin anomalad, which also has been diagnosed prenatally,9 the dominant and recessive forms of the mandibulofacial dysostosis, or Treacher-Collins syndrome,10,11 and mandibuloacral dysplasia.12 Among skeletal dysplasias, a variable degree of mandibular hypoplasia can be present in campomelic dysplasia13 and in rarer syndromes such as the femorofacial syndrome.14 However, the group of diseases that accounts for most cases of micrognathia is the chromosomal aberration group. In fact, micrognathia has been reported in 80% of cases of trisomy 18 and triploidy at autopsy,1,2 and, conversely, an abnormal karyotype has been found in 66% of fetuses with micrognathia. This finding was reported in 1993 by Nicolaides et al,5 who studied 56 cases of micrognathia detected prenatally, which, to the best of our knowledge, represent the largest series of fetuses with micrognathia published to date. In that study, the authors were able to detect micrognathia by subjective evaluation of the fetal profile in 25% of fetuses with trisomy 18 in comparison with the 80% rate reported in an autopsy series.1 On the basis of this discrepancy, the authors stated that "at present only the most severe degrees of micrognathia are amenable to prenatal diagnosis." Since then, several efforts have been made to improve the ultrasonic diagnosis of fetal micrognathia, which relied on the subjective evaluation of the profile on a sagittal midline scan of the fetal face. To render the diagnosis of micrognathia more objective, the length of the mandibular bone was measured, and growth curves, showing a linear relationship with gestational age and biometric parameters, were developed.3,4 However, neither Otto et al3 nor Chitty et al4 have further supported their work with a prospective study. In the present study, we confirmed the linear relationship with gestational age and BPD for both the anteroposterior and the laterolateral diameter (Table 1Go). In addition, it has been shown in the prospectively studied population of 198 malformed fetuses that the measurement of the jaw index has a greater diagnostic accuracy than the subjective assessment of the facial profile (Table 4Go). In particular, in the diagnosis of micrognathia, a jaw index of less than 23 has a 100% sensitivity and 98.7% specificity and a jaw index of less than 21 has a 100% positive predictive value (Table 3Go). In previous studies,3,4 the mandibular growth process was assessed in a single plane, measuring the length of the bone from the symphysis mentis to the proximal end of the ramus. In the present study, the measurement of the two orthogonal diameters (laterolateral and anteroposterior) allowed us to analyze separately the two vectors of mandibular growth (coronal and sagittal). This method has shown that the lateral growth of the mandible is relatively regular also in cases of micrognathia and that the growth process is impaired primarily in the antero-posterior plane (Figures 2Go and 3Go). We think that this consideration might explain the relatively high diagnostic performance of the anteroposterior mandibular measurement and, hence, of the jaw index.


   Footnotes
 
PII S0029-7844(98)00414-1

Received April 27, 1998. Received in revised form July 31, 1998. Accepted August 13, 1998.


   References
Top
Abstract
 Materials and Methods
 Results
 Discussion
 References
 
1. Buyse ML. The birth defects encyclopedia. Boston, Massachusetts: Blackwell, 1990:1099–102.

2. Jones KL. Smith’s recognizable patterns of human malformation. 5th ed. London: WB Saunders, 1997:794–5.

3. Otto C, Platt LD. The fetal mandible measurement: An objective determination of fetal jaw size. Ultrasound Obstet Gynecol 1991; 1:12–7.[Medline]

4. Chitty LS, Campbell S, Altman DG. Measurement of the fetal mandible—Feasibility and construction of a centile chart. Prenat Diagn 1993;13:749–56.[Medline]

5. Nicolaides KH, Salvesen DR, Snijders RJM, Gosden C. Micrognathia fetal facial defects: Associated malformations and chromosomal abnormalities. Fetal Diagn Ther 1993;8:1–9.

6. Laitinen SH, Ranta RE. Cephalometric measurements in patients with Pierre-Robin syndrome and isolated cleft palate. Scand J Plast Reconstr Surg Hand Surg 1992;26:177–83.[Medline]

7. van der Haven I, Mulder JW, van der wal KGH, Hage JJ, de Lange-de Klerk ESM, Hauman TJ. The jaw index: New guide defining micrognathia in newborns. Cleft Palate Craniofac J 1997; 34:240–1.[Medline]

8. Macpherson TA, Study group for complications of perinatal care (SGCPC). A model perinatal autopsy protocol. AFIP (Armed Forces Institute of Pathology), ed. Washington, DC, 1994:234–6.

9. Pilu G, Romero R, Reece EA, Bovicelli L, Hobbins JC. The prenatal diagnosis of Robin anomalad. Am J Obstet Gynecol 1986;154: 630–2.[Medline]

10. Franceschetti A, Klein D. Mandibulo-facial dysostosis: New hereditary syndrome. Acta Ophthalmol 1949;27:141–224.

11. Reynolds JF. A new autosomal dominant acrofacial dysostosis syndrome. Am J Med Genet, 1986;25Suppl 2:143–50.

12. Young LW. New syndrome manifested by mandibular hypoplasia, acro-osteolysis, stiff joints, and cutaneous atrophy (mandibuloacral dysplasia) in two unrelated boys. BD:OAS VII(7). New York: March of Dimes Birth Defects Foundation, 1971:291–7.

13. Turner GM, Twining P. The facial profile in the diagnosis of fetal abnormalities. Clin Radiol 1993;47:389–95.[Medline]

14. Robinow M, Sonek J, Buttino L, Veghte A. Femoral-facial syndrome—prenatal diagnosis—autosomal dominant inheritance. Am J Med Genet 1995;57:397–9.[Medline]




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