Several recent studies reported the sensitivity, specificity, and PPV of NIPT for T21, T18, and T13 screening. Yet, so far, no large-scale clinical studies have been conducted to assess the efficiency of NIPT for detecting SCA. To the best of our knowledge, this is the largest study (45773 cases) that evaluated NIPT for SCA. According to our data, the overall PPV of SCA detected by NIPT was 40.56%, within the rage of PPV for SCA screening between 30–60%[12, 14, 17, 18, 19]. When categorized by individual SCAs, the PPVs were 12.5% for 45,X, 51.72% for 47,XXX, 66.67% for 47,XXY, and 83.33% for 47,XYY, which was similar to those reported by other studies[1, 12, 20]. Yet, in our study, we noticed that different pregnancies characteristics showed different PPV, e.g., the PPV of SCA in AMA pregnant women (50.79%) was higher than in the total samples (40.56%). Therefore, we believe that AMA is a high-risk factor for SCA. Moreover, the overall termination rate was 50% for SCA (including mosaic cases), and 41.38% (not including mosaic cases); the termination rate for fetal SCA was 85.71% for 45,X, 20% for 47,XXX, 73.08% for 47,XXY, and 10% for 47,XYY.
Our study showed that NIPT performed better in predicting sex chromosome trisomies than monosomy X. This may be due to the following: (1) there are 1098 genes on the X chromosome and 78 genes on the Y chromosome; 58 genes are homologous genes on both sex chromosomes. The majority of the genes (29 genes) are at the end of the sex chromosomes. (2) the low guanosine-cytosine content of the X chromosome leads to highly variable amplification of the X chromosome. (3) the non-random inactivation of the X chromosome in placental tissue might be the reason for the low PPV of Turner syndrome, with the paternal X chromosome tending to inactivate in XX female trophoblasts[1, 12]. Besides, it was reported that there is an age-related X chromosome loss in normal female white blood cells, which may influence the effectiveness in predicting fetal 45,X; although this was not observed in our study.
The PPV of SCA is lower than other common chromosome aneuploidy. The reason is that sex chromosome abnormalities are less prevalent. Wang et al reported that 8.6% positive results for SCA were due to maternal mosaicism, which was later confirmed by other studies[1, 12, 22]. With the combination of NIPT and maternal peripheral blood karyotype analysis, we found that about 12.5% of the discordant NIPT SCA results are due to maternal mosaicism in our study. Previous studies have demonstrated that the identification of maternal karyotype tends to decrease the rate of false-positive SCA and can offer an explanation for the false-positive results for SCA[12, 13, 23]. With regard to the discordance between NIPT and invasive prenatal testing, another reason is confined placental mosaicism, which occurs approximately in 1–2% of all pregnancies. The origin of most cell-free fetal DNA in the maternal plasma is mostly from the apoptosis of placental cells from the cytotrophoblast. The mosaicism degree reduces the effective cell-free fetal DNA concentration in maternal plasma, thus affecting the performance of NIPT in detecting fetal aneuploidies. Our results verified the value of determining the maternal karyotype in increasing the accuracy of reporting NIPT results for chromosomes X and Y; however, further studies are needed to provide more clinical data in support of this premise.
With the prenatal diagnosis of fetal SCA, some pregnant women were willing to continue their pregnancy, and there were differences in the rate of pregnancy termination rate between different types of SCAs. Pregnant women with fetuses of 45,X and 47,XXY were more eager to terminate a pregnancy than those with 47,XXX and 47,XYY, which was consistent with other studies[12, 25]. Gruchy et al reported that with regard to Turner syndrome, the rate of pregnancy termination was closely related to the 45,X karyotype, mosaic karyotype, and structural abnormalities of the X chromosome. In our study, pregnant women with a fetus of 45,X with the mosaic of 23.08% had a stronger tendency to continue the pregnancy, while those with a mosaic of 28.38%, 48.53%, 59.38%, 76.67%, and 80.49% were more inclined to terminate a pregnancy. Nowadays, studies report that almost all pregnant women carrying fetuses with 45,X decide to terminate their pregnancy. To some extent, the parental decisions for the pregnancy termination may have been related to the types of SCA, level of prenatal genetic counseling, history of infertility, parental and social acceptance, economic condition, and similar.
In clinical practice, AMA pregnant women are willing to accept NIPT as a non-invasive and accurate screening. In this study, the proportion of AMA pregnant women was the second-highest in the NIPT screening population, reaching 36.97%. Among them, NIPT detected 108 cases SCA; 63 cases underwent amniocentesis, and 32 (50.79%) cases were confirmed to be true-positive. Of all the confirmed SCA cases, 55.17% (32/58) were from AMA pregnant women. The study showed that NIPT could be used as a useful screening test for SCA in AMA pregnant women, which was similar to the results reported by Zheng et al . The differences in the frequency of SCA were statistically significant among the age groups, and the frequency was significantly higher in the > 39 years age group (P < 0.05). Therefore, genetic counseling, combined with serological screening tests and B-ultrasound detection of abnormalities, should be fully carried out for AMA pregnant women. The frequencies of 47,XXX and 47,XXY were significantly correlated with maternal age, whereas frequencies of 45,X and 47,XYY did not show significant correlations. Although there was no statistical significance between the frequency of 47,XYY and the maternal age, the frequency of 47,XYY decreased with maternal age in the advanced aged group. To determine whether there is a correlation between the maternal age and the frequency of fetus 47,XYY, further studies with bigger sample size are warranted. The risk of 47,XXX increased with AMA, which was consistent with Zhu et al study. Previous studies showed that for 45,X syndrome, the maternal age coefficient is negative and implies a decreasing incidence in older mothers. Yet, this was not consistent with our results. Understanding the frequency of SCA has proven valuable in counseling couples who seek advice about the risk of fetal sex chromosome abnormalities with AMA and are considering the option of prenatal diagnosis and termination of pregnancy.
This study has a few limitations. First, the sensitivity, specificity, and negative predictive value were not calculated. Newborns with SCA usually appear phenotypically normal. Therefore, it was difficult to confirm the results of SCA without karyotype analysis during their neonatal period. Second, the number of SCA cases in our study was not big enough to discuss its frequency across different age groups. Therefore, more pregnancies must be evaluated in order to further understand the association of maternal age with fetal SCA.
Recent studies have demonstrated that early interventions such as postnatal hormone therapy, physical therapy, and occupational therapy could have positive effects on the behavioral phenotype or neurodevelopmental outcomes if applied earlier to SCA patients[9, 29]. Prenatal screening and diagnosis of SCA can provide the opportunity for early intervention, comprehensive postnatal management, and improve the quality of life of the affected child. Sex chromosome abnormalities are more common than the major trisomies at birth, and the neonates are often phenotypically normal. Conventional prenatal screening cannot be used to directly identify sex chromosome abnormalities that can only be identified using postnatal karyotyping, which in turn may delay the treatment of SCA patients. NIPT allows prenatal screening of SCA. The application of NIPT can provide an alternate option for pregnant women to invasive prenatal testing for the identification of fetal sex chromosome abnormalities. However, there are still some issues that require further consideration. Due to the existing false positive rate of NIPT screening for SCA, the number of unnecessary invasive prenatal diagnosis may increase, especially for 45,X. Still, the benefit of detection for fetal SCA outweighs the risk related to invasive procedures. Some pregnant women may decide to terminate a pregnancy if the chromosomal abnormalities are accidentally discovered by SCA screening, which involves ethical issues regarding the mild phenotype of SCA and the potential increase in the rate of gender selection.