almost a century ago , a linear correlation was observed between gestational age ( ga ) and foot length ( ft ) in 704 human fetal specimens from around 50 days postconception until birth.1 the correlation has been confirmed by studies in utero,26 in abortus,68 or in stillborn,9,10 and in neonates to other measurements.11 normograms for the first trimester and later gestation have been constructed for assessment of ga . however , there is less information on the correlation of fetal ft to the other parameters in the early part of pregnancy . this information may be helpful when fetal abnormality is suspected at an early gestation.12,13 in this paper , we explore the relationship of fetal ft with the other fetal measurements at 1016 weeks gestation . during the period from march 7 , 2014 to september 7 , 2016 , women attending an obstetric clinic with normal pregnancies were recruited . a transabdominal ultrasound scan was performed as a part of antenatal assessment ( accuvix v20 prestige , medison with 48 mhz volumetric transducer or epiq 7 , philips with x6 - 1 matrix transducer ) . fetal measurements in millimeters were taken , including biparietal diameter ( bpd ) , head circumference ( hc ) , abdominal circumference ( ac ) , femur length ( fl ) and ft . the crown rump length ( crl ) cases were excluded if the entire foot could not be clearly visualized during the ultrasound examination . the fetal ft was taken as the longest distance from the most posterior point of the foot to the tip of the first or the second toe , whichever was longer ( figure 1 ) . the estimated ga in weeks was calculated from the last normal menstrual period or from the first trimester dating scan if there was a discrepancy of more than a week . the ultrasound examination was performed as a part of routine antenatal assessment to the patients understanding . therefore , no written consent was signed and the patients remained anonymous in the study . although the fetal measurements were taken prospectively , this was a retrospective analysis involving negligible risk . it conformed to the standards established by the australian government national health and medical research council ( nhmrc ) not requiring ethical review ( clauses 2.1.7 , 5.1.22 , 5.1.23 ) , and ethics approval was not sought within the institution.14 results were analyzed with statistical package for the social sciences version 17.0 ( spss inc . , chicago , il , usa ) . a two - sided probability ( p ) value of < 0.05 was considered statistically significant . the mean and standard deviation of their age , gravidity and parity were 32.24.3 years , 2.31.5 and 0.70.7 , respectively . the fetal ft ( mm ) is plotted against the ga ( weeks ) , as shown in figure 2 . the bpd , hc , ac , fl and crl ( mm ) are plotted against ft in figures 37 , respectively . a linear relationship was observed between ft and bpd , hc , ac , fl and crl ( p<0.001 ) . the graphs of fetal fl to ft ratio against the ft and the ga are shown in figures 8 and 9 , respectively . the graph of the ratio of fl to ac against the ga is shown in figure 10 . the regression models for figures 27 are shown within the legend for each figure and are summarized in table 1 . the pearson coefficient is 0.992 for intraobserver correlation ( p<0.001 ) and 0.990 for interobserver correlation ( p<0.001 ) in the measurement of fetal ft . the fetal ft showed a linear correlation with ga , bpd , hc , ac , fl and crl between 10 and 16 weeks gestation ( p<0.001 ; figures 27 ) . the correlation was the highest with hc , with the adjusted r being 0.950 , followed by bpd ( 0.945 ) , ac ( 0.943 ) , fl ( 0.937 ) , ga ( 0.816 ) and was the least with crl ( 0.798 ; table 1 ) . this is consistent with the histologic findings that the fetal ft bears a linear relationship with the ga , even at an earlier gestation.15 as fetal ft is noted to be independent of gender , environmental tobacco smoke , maternal smoking and alcohol consumption in human abortus up to 69 days postconception15 ( or below 12 weeks by last normal menstrual period ) , it could be used to estimate the fetal ga , even at an early gestation , as observed in previous studies.1,2,8 the ga could be estimated from the ft by the following equation ( figure 2 ; table 1 ) : 
 ga=0.335ft+8.845(adjustedr2=0.816,p<0.001 ) among all fetal parameters measured in our study , the correlation was the best between the fetal ft and hc . the hc could be estimated from the ft measurement by the following equation ( figure 4 ; table 1 ) : 
 hc=4.407fl+31.324(adjustedr2=0.950,p<0.001 ) all the fetal measurements are in millimeters and the estimated ga is in weeks . in our study , crl , though showing a linear correlation with the fetal ft , had the least correlation with the fetal ft among the measured fetal parameters ( table 1 ) . it is known that there could be variation of crl measurement as a result of fetal flexion or extension . although , in current practice , crl is commonly used to assess the ga in early pregnancy , a combination with other parameters , for example , bpd , hc or ft , could be helpful in increasing the accuracy . the fetal fl to ft ratio of 1 is often used in prenatal diagnosis of fetal skeletal dysplasia.16 in this study , the fetal fl to ft ratio increased with the ft ( figure 8) or the ga ( figure 9 ) in an inverse relationship ( r=0.49 , p<0.001 and r=0.516 , p<0.001 , respectively ) . it approached 0.9 when the ft was around 25 mm ( figure 8) or beyond 17 weeks gestation ( figure 9 ) . therefore , one would have to refer to the ga - related fl to ft ratio when fetal abnormality is suspected in early gestation ( eg , down syndrome17 or skeletal dysplasia13 ) , instead of using a uniform figure of 1 which researchers have established for later gestation.18 the fl to ac ratio , a parameter also used for prenatal assessment of skeletal dysplasia,13 was found to increase with the ga in early pregnancy in this study as well , and reached 0.16 only beyond 15 weeks gestation ( figure 10 ) . our results also correlated well with the results of other studies.3,15 this is a retrospective analysis , although the data have been collected prospectively . the fetal ft correlated linearly with other fetal measurements including bpd , hc , ac , fl , crl and estimated ga at 1016 weeks gestation . it could be used alone or in combination with other fetal parameters for assessment of fetal ga . a combination of fetal parameters may be better than crl alone in the assessment of fetal ga in early pregnancy . caution is required when using the fl to ft ratio and fl to ac ratio for the assessment of fetal abnormality in this period , as these are not of a fixed value but rather increase with the ga .
objectivesto present the fetal measurements in relation to the foot length at 1016 weeks gestation on antenatal ultrasound.materials and methodswe reviewed the fetal measurements in 47 scans performed at 1016 weeks gestation in women attending routine antenatal care . 
 the fetal measurements and measurement ratios were examined in relation to the foot length.resultsthe fetal biparietal diameter , head circumference , abdominal circumference , femur length and foot length increase linearly with the gestational age at 1016 weeks gestation . 
 the correlation between fetal foot length and fetal biparietal diameter , head circumference , abdominal circumference and femur length are better than with the crown rump length . 
 the femur length to foot length ratio and the femur length to abdominal circumference ratios increase with the gestational age in an inverse relationship throughout the same period.conclusionfetal foot length is an accurate estimate of the gestational age in early pregnancy . compared with crown rump length , the correlation between foot length and other fetal measurements is better . 
 the fetal measurements ratios are gestational age dependent between 1016 weeks . 
 gestational age specific ratio should be used in this period .