Accurate assessment of gestational age (GA) is fundamental in obstetrics, crucial for both individualized patient care and population health monitoring. For individual patients, precise dating ensures appropriate antenatal care, timely interventions, and improved perinatal outcomes [1]. At the population level, it supports reliable surveillance of GA-dependent outcomes such as preterm birth and small-for-gestational-age neonates [1]. Traditionally, the last menstrual period (LMP) has been used to estimate GA; however, this method is limited by recall bias, irregular cycles, and variability in ovulation. Up to 45% of women are uncertain of their menstrual dates, and LMP-based dating carries a 95% prediction interval of ±4.65 weeks [1]. Consequently, ultrasonography has become the gold standard for GA estimation due to its superior accuracy and reproducibility. Commonly employed biometric parameters include crown–rump length (CRL), mean gestational sac diameter (MGSD), biparietal diameter (BPD), head circumference (HC), femur length (FL), and abdominal circumference (AC) [2]. CRL is the most accurate parameter in the first trimester, with an error margin of ±4 days, while MGSD is accurate within ±10 days [3]. In the second and third trimesters, BPD, HC, FL, and AC are widely used, though their precision decreases as gestation advances. For example, BPD estimates GA within ±10 days in the second trimester but ±21 days in the third, whereas FL is accurate to within ±6 days and ±14 days, respectively [2,3]. To improve reliability, multiple biometric parameters are recommended in later pregnancy. The fetal interorbital distance (IOD)—the distance between the inner margins of the orbits—has emerged as a promising additional parameter for GA estimation. Orbital biometry was first described by Mayden et al. as a reliable indicator of fetal maturity, while Turner and Twining demonstrated its utility in animal studies [2,4]. Beyond GA assessment, IOD also aids in the prenatal diagnosis of craniofacial anomalies such as hypertelorism, hypotelorism, cyclopia, and congenital cataracts [5]. Unlike BPD and HC, IOD is relatively unaffected by variations in skull shape, making it a potentially stable predictor of GA. Craniofacial morphology exhibits racial and ethnic variability, with differences in skull size and orbital dimensions reported across populations [2,6,7]. This underscores the need for population-specific reference values. Nevertheless, data on fetal IOD in the local population remain limited, particularly regarding its role in GA prediction. This study aims to evaluate interorbital distance as a predictor of gestational age in the second and third trimesters and to determine its correlation with femur length, one of the most consistent and established biometric parameters. Validating this correlation may establish IOD as an adjunctive tool in fetal biometry, thereby enhancing the accuracy and reliability of GA estimation.

Study Setting and Design This observational cross-sectional study was conducted in the Department of Radiodiagnosis, Career Institute of Medical Sciences and Hospital, Lucknow, over a period of two years. Study Population Pregnant women in their second and third trimesters (13–36 weeks gestation) attending the radiology department were considered for inclusion. Inclusion Criteria • Singleton pregnancies between 13- and 36-weeks’ gestation, confirmed by last menstrual period (LMP) and ultrasound. • Women willing to provide written informed consent. Exclusion Criteria • Multiple pregnancies. • Pregnancy-related complications, including pre-eclampsia, gestational diabetes mellitus, or intrauterine growth restriction (IUGR). • Presence of congenital malformations or high-risk pregnancies detected on ultrasound. Sampling Technique and Sample Size Participants were recruited using a convenience sampling (non-probability) method. The sample size was calculated for cross-sectional studies according to the formula described by Pala HG et al., [8] considering a standard deviation (SD) of 0.24 and an absolute error (d) of 3% (0.03). A total of 270 participants were enrolled. Procedure Written informed consent was obtained from all participants after explaining the study. During the ultrasound examination, participants were positioned supine with the abdomen exposed. A water-based ultrasound gel was applied, and fetal biometric parameters—including interorbital distance (IOD) and femur length (FL)—were measured using a curvilinear probe according to standard protocols. All measurements were performed by experienced sonographers to minimize interobserver variability. Each parameter was measured at least twice, and the mean value was recorded. Ultrasound machine settings and probe frequency were standardized across all examinations to ensure reproducibility. Standard anatomical landmarks were used for IOD and FL measurements. Gestational Age Calculation Gestational age was independently estimated using IOD and FL. Standard reference charts were used to convert biometric measurements into gestational age. The correlation between IOD and FL was analyzed to assess the accuracy of IOD in predicting gestational age during the second and third trimesters. Ethical Considerations The study was approved by the Institutional Ethics Committee of Career Institute of Medical Sciences and Hospital, Lucknow. All procedures adhered to the Declaration of Helsinki, and participant confidentiality was maintained throughout the study. Statistical Analysis Data were analyzed using SPSS software version 17 (IBM Corp., Armonk, NY, USA). Continuous variables were expressed as mean ± standard deviation (SD), and categorical variables as frequencies and percentages. Pearson’s correlation coefficient (r) assessed relationships between interorbital distance (IOD), femur length (FL), other fetal biometric parameters (BPD, HC), and gestational age (GA). Multiple linear regression identified independent predictors of GA, with GA as the dependent variable and IOD, FL, BPD, and HC as predictors; model fit was evaluated using adjusted R² and standard error of estimate (SEE). A p-value < 0.05 was considered statistically significant. Scatter plots were used to graphically illustrate correlations between GA and fetal biometric parameters.

A total of 270 pregnant women were included in the study, with a mean maternal age of 34.61 ± 5.32 years. The cohort was predominantly nulliparous (55.9%) and had a mean body mass index (BMI) of 24.16 ± 2.69 kg/m². Gestational age (GA) distribution peaked between 25 and 28 weeks (31.5%). The mean fetal biometric measurements were: femur length (FL) 4.75 ± 1.39 cm, interorbital distance (IOD) 0.44 ± 0.09 cm, biparietal diameter (BPD) 6.48 ± 1.59 cm, and head circumference (HC) 24.40 ± 5.74 cm. All parameters demonstrated highly significant correlations with GA (p < 0.001). Pearson correlation analysis revealed strong associations between GA and established biometric parameters: FL (r = 0.993), BPD (r = 0.982), and HC (r = 0.992). IOD also showed a strong correlation with GA (r = 0.964) and significant correlations with other fetal growth markers: FL (r = 0.959), BPD (r = 0.961), and HC (r = 0.969), all p < 0.001. Multiple linear regression analysis produced an optimized predictive model (adjusted R² = 0.992, standard error of estimate [SEE] = 0.5145). FL, BPD, and HC were identified as independent predictors of GA (p < 0.01), whereas IOD did not reach statistical significance in the multivariate model (p = 0.532). These findings indicate that while IOD is strongly correlated with GA and conventional fetal biometric parameters, its independent predictive value is limited compared to FL, BPD, and HC. Nevertheless, IOD may serve as a useful adjunct in fetal biometry, particularly in situations where standard measurements are difficult to obtain. These findings demonstrate that while interorbital distance (IOD) exhibits strong bivariate correlations with gestational age (GA) and established fetal biometric parameters, its utility as an independent predictor is limited compared to femur length (FL), biparietal diameter (BPD), and head circumference (HC). The robust performance of FL, BPD, and HC underscores their primary role in accurate GA estimation, as reflected by the regression model’s near-perfect explanatory power. Nevertheless, the consistent associations between IOD and other biometric measures suggest that it may serve as a valuable adjunct, particularly in clinical scenarios where conventional measurements are challenging or when detailed craniofacial assessment is required. These results highlight the importance of a multimodal approach to fetal biometry and emphasize the need for further research to define the clinical applicability of IOD in specialized diagnostic contexts.

Accurate determination of gestational age (GA) is essential in obstetric care, as erroneous estimates are associated with adverse maternal and neonatal outcomes [9]. Although the last menstrual period (LMP) has traditionally been used, approximately 40% of pregnancies have uncertain LMP dates, making sonographic assessment more reliable [7]. First-trimester crown–rump length (CRL) provides accuracy within ±4 days, whereas second- and third-trimester parameters—biparietal diameter (BPD), femur length (FL), and head circumference (HC)—exhibit variable precision (±6–21 days). Combining multiple parameters (BPD, FL, HC, and abdominal circumference) improves GA estimation after 20 weeks of gestation. In this study, interorbital distance (IOD) was evaluated as a potential predictor of GA in 270 singleton pregnancies (13–36 weeks) using fronto-transverse sonographic measurements. The cohort had a mean maternal age of 34.61 ± 5.32 years and a mean BMI of 24.16 ± 2.69 kg/m². Mean fetal biometric measurements were: IOD 0.44 ± 0.09 cm, FL 4.75 ± 1.39 cm, BPD 6.48 ± 1.59 cm, and HC 24.40 ± 5.74 cm. Pearson correlation analysis demonstrated strong bivariate associations between IOD and conventional parameters: FL (r = 0.959), BPD (r = 0.961), and HC (r = 0.969), all p < 0.001. IOD also correlated strongly with GA (r = 0.964). Multiple linear regression identified FL (B = 0.216), BPD (B = 0.316), and HC (B = 0.379) as independent predictors of GA (p < 0.01), whereas IOD was not independently predictive (B = 0.088, p = 0.532), despite excellent model fit (adjusted R² = 0.992). These results align with prior studies reporting IOD’s growth trajectory and utility in detecting craniofacial anomalies such as hypertelorism and hypotelorism. Research by Nwadike et al. [7] in Nigeria and Gareeballah et al. [10] in Sudan further supports IOD’s reliability when standardized measurement techniques are employed. Nevertheless, conventional parameters—FL, BPD, and HC—remain superior for GA estimation. IOD may serve as a complementary measurement, particularly when standard parameters are limited or detailed craniofacial assessment is required. Similar trends have been reported in South Asian populations, including Bangladesh [11], and in Western populations, suggesting minimal ethnic or racial variation in orbital parameters [12–14]. Predicted fetal orbital biometric parameters, including BOD, OD, and IOD, correspond closely with previously reported values [11,15, 16]. Study limitations include restriction to singleton pregnancies, operator-dependent measurement variability, and exclusion of first-trimester assessments. Strengths include the novel evaluation of IOD, non-invasive methodology, and potential applicability across populations with variable craniofacial morphology. Moreover, IOD demonstrates strong correlations with GA and conventional fetal biometric parameters, supporting its use as an adjunct in fetal biometry. Incorporating IOD into multimodal GA assessment may enhance accuracy, particularly in challenging scenarios. Future research should focus on standardized measurement protocols, first-trimester applications, and population-specific reference charts to validate its clinical utility.

In this study, interorbital distance (IOD) demonstrated a strong correlation with gestational age (GA) and established fetal biometric parameters, including femur length (FL), biparietal diameter (BPD), and head circumference (HC). While IOD did not emerge as an independent predictor of GA in multivariate analysis, its consistent associations suggest that it can serve as a valuable adjunct in comprehensive fetal assessment. Incorporating IOD alongside conventional parameters may improve GA estimation, particularly when standard measurements are limited or challenging. These findings support a multimodal approach to fetal biometry and highlight the potential clinical utility of IOD in prenatal evaluation. Further research is warranted to establish population-specific reference values and standardized measurement protocols to facilitate its integration into routine clinical practice.

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