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Ultrasound gender study

how accurate are fetal
sex determination scans

early ultrasound determination of fetal sex
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boygirlWhats the accuracy of sonographic
determination of fetal gender study

  
The objective of this study was to examine the accuracy of fetal gender prediction at a routine first trimester scan. Pregnant women, from an unselected population around the world were recruited for this study. They agreed to the study, to examine the accuracy of fetal gender prediction, at a routine first trimester scan for detailed assessment of fetal anatomy and nuchal thickness measurement.

The clinical value of early ultrasound determination of fetal sex includes, confirmation of zygosity and analysis of chorionic villous sampling in twin pregnancies, early information for demanding parents and a powerfull method to decide whether to carry out prenatal invasive testing in pregnancies at risk of sex-linked genetic disorders, because it would be unnecessary in pregnancies with female fetuses.

The examinations were made with the use of normal two-dimensional (2D), three-dimensional (3D), transabdominal and transvaginal sonography data. Sex confirmation data was obtained postnatally from hospital registries, parents information or by ultrasound performed after 25 weeks of gestation.

Some gender determination was performed as a part of the sonographic examination preceding genetic amniocentesis or chorionic villus sampling. The results were compared with the gender at birth or with fetal karyotype results obtained from amniotic fluid cells or chorionic villus sampling.

What is a sagittal view? (vertical cut away slice of the body as seen from its side)

Details of 'Angle of the 30 degrees' study

The fetal gender was assigned as male if the angle of the genital tubercle to a horizontal line through the lumbosacral skin surface (lower portion of the spine) was greater than 30 degrees and female when the genital tubercle was parallel or convergent (less than 10 degrees) to the horizontal line.

12 week old fetus - (Fetus size = 3 inch, 75 mm) - 14 weeks pregnant

Male gender
Male 11weeks		         Female gender
Female 11weeks

In some of the study groups, when the angle of the genital tubercle was an intermediate angle of 10 to 30 degrees the fetal gender was not determined.

Out of a total of 1619 pregnancies; gender was assigned and confirmed in 1424. The table below shows the results in so far for accuracy of fetal gender prediction at a routine first trimester scan.

Results of 'Angle of the 30 degrees' studies

Weeks of gestation11 weeks12 weeks13 weeks14 weeks
Ability to assign fetal
gender % age group		68%88%94%98%
Correct identification
fetal gender		50%84%90%94%
Male fetuses incorrectly
assigned as female		41%13% 8% 5%
Female fetuses incorrectly
assigned as male		 9% 3% 2% 1%
Gender correct from
100 baby scans		34808592


CONCLUSION:
Prenatal gender assignment by ultrasound has a high accuracy rate at 12 to 14 weeks. At 11 weeks there was an error rate of 50% and only 14/100 assigned correct male gender. In the male fetuses after 12 weeks, there was a significant increase in the angle of the genital tubercle from the horizontal. The accuracy of sex determination increased with gestation.

Results from: Brazil  |  UK (Group1)  |  UK (Group2)  |  Israel


Details of Embryo genital development
 
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STUDY GROUP:
  • Brazil - 267 fetuses at 11 to 13+6 weeks of gestation including singleton and twin pregnancies.


    • METHODS:
  • Transabdominal scan was performed and examined the genital region in a sagittal plane of the fetus


    • RESULTS:
  • Fetal gender identification was successfully achieved in 97% (259/267) of the study group.

  • The accuracy of sex determination (positive predictive value) increases from 74.2% at 11 weeks, trough 90.5% at 12 weeks to 95.7% at 13 weeks.


  • Male fetuses were incorrectly assigned as female in 29.2% of cases at 11 weeks, 12.2% at 12 weeks and 5% at 13 weeks.


  • Female fetuses were incorrectly assigned as male in 14.3% of cases at 11 weeks, 4.5% at 12 weeks and 3.3% at 13 weeks.


Our results suggest that first trimester sex determination by routine ultrasound reaches the best accuracy at the 13th week of gestation.

 
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STUDY GROUP:
  • London UK (Group 1) - A total of 524 women at 11 to 14 weeks of gestation.


    • METHODS:
  • Transabdominal sonography and transvaginal sonography (26%) when necessary and examined the genital region in transverse and sagittal plane of the fetus. At an intermediate angle of 10 to 30 degrees the gender was not determined.


    • RESULTS:
  • The ability to assign fetal gender significantly improved with increasing gestational age, being 59%, 87%, 92% and 98% at 11, 12, 13 and 14 weeks, respectively.

  • The overall success of correctly assigning fetal gender (positive predictive value) increased with gestational age from 46% to 75%, 79% and 90% at 11, 12, 13 and 14 weeks, respectively.


  • The abilty to correctly identify fetal gender when attempted did not change with gestational age.


  • Fetal gender or the performance of the scan by different operators did not affect the results.


Whilst the accuracy of sonographic determination of fetal gender at 11 to 14 weeks is good, it still falls significantly short of invasive karyotyping tests.

 
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STUDY GROUP:
  • London UK (Group 2) - 172 singleton pregnancies at 11 to 14 weeks of gestation before chorionic villus sampling for karyotyping.


    • METHODS:
  • Transabdominal sonography and transvaginal sonography and examined the genital region in transverse and sagittal plane of the fetus.


    • RESULTS:
  • The accuracy of sex determination increased with gestation from 70.3% at 11 weeks, to 98.7% at 12 weeks and 100% at 13 weeks.

  • Male fetuses were wrongly assigned as female in 56% of cases at 11 weeks, 3% at 12 weeks and 0% at 13 weeks.


  • In contrast, only 5% of the female fetuses at 11 weeks were incorrectly assigned as male and this false-positive rate was 0% at 12 and 13 weeks.


  • In the male fetuses, there was a significant increase in the angle of the genital tubercle from the horizontal with crown-rump length.


Our results suggest that a final decision on invasive testing for sex-linked conditions should be undertaken only after 12 weeks of gestation.

 
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STUDY GROUP:
  • Tel Aviv, Israel - 656 singleton pregnancies at 12 to 14 weeks of gestation. Phenotypic sex was confirmed in 555 newborns.


    • METHODS:
  • Fetal gender assessment by transabdominal ultrasound and examined the genital region in sagittal plane of the fetus. At an intermediate angle of 10 to 30 degrees the gender was not determined.


    • RESULTS:
  • The ability to assign fetal gender significantly improved with increasing gestational age, being 57%, 86%, 92% and 97% at 11, 12, 13 and 14 weeks, respectively.

  • Gender identification according to CRL was feasible in 85%, 95% and 97% of the fetuses at gestational ages of 12 to 12 + 3, 12 + 4 to 12 + 6 and 13 to 13 + 6 weeks, respectively.


  • The accuracy of male gender assignment in this group was 99 to 100% at all ages,


  • and that of female gender assignment was 91.5% at 12 to 12 + 3 weeks, 99% at 12 + 4 to 12 + 6 weeks and 100% at 13 to 13 + 6 weeks.


Prenatal gender assignment by ultrasound has a high accuracy rate at 12 to 14 weeks. These results indicate that invasive testing can probably be carried out in fetuses identified as males at this gestational age. However, in fetuses identified as female at a CRL of greater 62.6 mm, despite the relatively high 91.5% accuracy rate, the decision regarding invasive testing should be postponed until a higher CRL is achieved.

 
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