Objective: To evaluate the accuracy of fetal sex determination using ultrasonography performed by Thai Maternal Fetal
Medicine fellowships during second trimester genetic amniocentesis and also identify the possible factors of inaccurate determination.
Material and Method: A prospective non-random study was conducted to evaluate the accuracy of fetal sex determination
using ultrasonography performed by Thai Maternal Fetal Medicine fellows at Songklanagarind Hospital during second
trimester genetic amniocentesis and also identify the possible factors of inaccurate determination.
Results: Five hundred and sixty singleton pregnant women were enrolled. Eight cases were excluded due to abnormal fetal karyotype. No pregnancy loss or fetal ambiguous genitalia presented. The sex of 11/552 (1.99%) fetuses could not be
adequately determined by ultrasonography. Overall, 491 of the 541 diagnose were correct – an accuracy of 90.8% (CI 88.0-
93.1) given an ultrasonographic diagnosis could be made. Among phenotypic females, the accuracy was 226/246, (91.9%; 95% CI 87.7-95.0) and among phenotypic males, the accuracy was 265/295 (89.8%; 95% CI 85.2-92.4). The ultrasonographic diagnosis of female sex was correct in 226/256, (88.3%; 95% CI 84.0-92.0), and the ultrasonographic diagnosis of male sex was correct in 265/285, (93.0%; 95% CI 89.4-95.7). For both sexes combined and for female fetuses, year of fellowship was only significant variable associated with correct sex determination by ultrasonography [2nd year vs. 1st year: overall ORS 2.55, (95% CI 1.44-4.61); female fetuses ORS 6.54, (95% CI 2.48-17.26)].
Conclusion: Fetal sex determination using ultrasonography by Thai Maternal Fetal Medicine Fellows at Songklanagarind Hospital during second trimester genetic amniocentesis is possible. Less experienced physicians should be had increased
awareness especially in case of female external genitalia diagnosis.

Keywords: Amniocentesis, Sex determination, Ultrasonography, Advance maternal age