Human sex ratio at amniocentesis and at birth in Taiwan

An increase in the proportion of male-to-female live births has raised concerns in Taiwan. Disclosure of fetal sex during prenatal screening is not allowed by the Taiwan government. Fetal sex annotation in clinical genetic reports is also prohibited. This study tested the hypothesis that the male-to-female sex ratio at amniocentesis should be lower than the sex ratio at birth, if a certain percentage of female fetuses are being selectively aborted after amniocentesis. Therefore, we examined the differences between fetal sex ratio at amniocentesis at a tertiary medical center in southern Taiwan and the nationwide sex ratio at birth in Taiwan from 1992 to 2011.

Data of normal male and female karyotypes during the study period were collected from the cytogenetic laboratory of the National Cheng Kung University Hospital (NCKUH) in southern Taiwan. Data of sex ratio at birth nationwide in Taiwan were obtained from the Department of Statistics, Ministry of the Interior, Taiwan. We calculated 95% binominal confidence intervals for the sex ratios and differences between fetal sex ratio at amniocentesis, and nationwide sex ratio at birth were tested by the χ(2) test and Bonferroni correction.

The nationwide sex ratio at birth ranged from 1.07 to 1.11 during the period from 1992 to 2011 in Taiwan, with the highest in 2004 and the lowest in 1993. The fetal sex ratio at amniocentesis at NCKUH ranged more widely (0.82-1.28), with the lowest in 1993 and the highest in 2007. After regression analysis, both trends of sex ratio at amniocentesis during midtrimester and at birth were not significantly increased by years. Furthermore, the sex distribution at amniocentesis during midtrimester did not differ significantly from the nationwide sex ratio at birth (1.113 vs. 1.092, p = 0.151).

The results showed that sex ratio was already skewed toward male at midtrimester. Our data imply that artificial sex selection, if it were present, might have already emerged prior to the timing of amniocentesis. However, more large nationwide studies on sex ratios in Taiwan are warranted.