Introduction
Spontaneous abortion is a common complication in human reproduction. An estimated 60% of spontaneous abortions occur before or after implantation (termed preclinical losses), while 10–15% are confirmed by ultrasound or histological evidence (termed clinical miscarriage) [
1,
2]. Most clinical miscarriages occur during the first trimester, and the leading cause of miscarriage is embryonal chromosomal abnormalities [
3,
4]. Although parental chromosomal structure abnormalities are the main genetic factor leading to recurrent miscarriages, the prevalence of chromosomal abnormalities in the affected couples is relatively low (2.78–4.1%) [
5‐
7]. Few studies have investigated the parental karyotype of sporadic miscarriage [
3]. The frequency of balanced rearrangements in the general population is very low (0.4%) [
1,
2,
8].
Apart from inheritable factors, a variety of maternal factors have been found to be related to spontaneous abortion or embryonic chromosomal aberrations, including age, reproductive history, and immune or endocrine dysfunction [
1‐
3,
9]. In addition, elevated maternal serum level of homocysteine was shown to increase the risk of fetal loss and stillbirth [
10]. Supplementation of folic acid can decrease the concentration of homocysteine and reduce the risk of pregnancy loss [
11,
12]. Polymorphisms in folate metabolizing genes are associated with chromosome breaks and fetal chromosomal aneuploidy [
13,
14].
Apart from maternal factors, embryonic chromosomal abnormalities may be attributable to abnormal gametogenesis in father. Sperm DNA fragmentation index tends to increase with paternal age [
15,
16]. Patients with high levels of sperm DNA damage showed a significantly higher miscarriage rate [
17]. Therefore, advanced paternal age may be associated with miscarriage, infertility, and birth defects [
18].
Other non-hereditary factors implicated in embryonic chromosomal aberrations may include environmental factors. Prenatal exposure to environmental factors, such as drugs and pesticides, may increase the risk of birth defects or embryonic DNA damage [
19,
20]. While several causes of teratogenesis have been identified, the etiopathogenetic mechanisms are not well characterized [
20‐
22]. There is a paucity of epidemiological evidence pertaining to the risk factors for embryonic chromosomal aberrations in cases of spontaneous abortion. Miscarriage is a distressing event for the affected women and their families. Therefore, exploring the etiopathogenesis of spontaneous abortion may help inform interventions to protect the developing embryo and prevent miscarriage. The aim of this study was to assess the risk factors for spontaneous abortions with and without embryonic chromosomal aberrations by investigating the clinical and demographic characteristics of these cases.
Discussion
Consistent with previous research [
3], most of the aborted embryos in this study had chromosomal abnormalities. Parental age may be an independent risk factor for fetal chromosomal abnormalities, especially for autosomal aneuploidies and complex aberrations. After adjusting for some potential confounding factors, the risk of embryonic chromosomal aberrations showed a positive correlation with paternal age, but not with maternal age. The reason may be that as the mother aged, the risk of fetal chromosomal aberrations increased, and the risk of unexplained spontaneous abortion increased as well [
27].
Similar to the earlier reports [
3,
28], the risk of polyploidy was not positively correlated with maternal age. Polyploidy, known as the endoreplication of genomic DNA [
29], has been found to increase the adaptive potential of organisms exposed to stressful conditions [
30]. However, polyploidization in embryonic DNA has serious consequences, and few can survive past the first trimester [
31].
The incidence of monosomy X showed no association with maternal age, which is consistent with previous reports [
3,
28]. Similarly, the incidence of monosomy X did not differ by paternal age, although in a previous study, monosomy X was found more likely to be caused by paternal chromosome loss [
32]. Nevertheless, we found that the serum level of Hcy in mothers with monomer X was higher than that in mothers with normal karyotype. In addition, the odds ratio for embryonic chromosomal abnormalities increased with the maternal serum Hcy level. Consistent with previous reports, high concentrations of Hcy may induce DNA damage and increase the risk of genomic instability in humans [
33]. Hcy is an intermediate product of folate metabolism; however, we found no correlation between folate supplementation and embryonic chromosomal aberrations. This may be attributable to inter-individual variability with respect to folate uptake and metabolism [
13]. In addition, we did not assess the timing of initiation of folic acid supplementation and the amount of supplementation, which may affect the effectiveness of folic acid supplementation [
12,
34].
There is no clear consensus on the association between obstetric history and embryonic karyotype. In a study by Ozawa et al., compared with women with < 2 previous miscarriages, women with ≥2 previous miscarriages had a lower incidence of chromosomal abnormalities in the aborted embryos [
3]. For parents with normal karyotypes, the incidence of abnormal embryo karyotypes decreased significantly with the number of previous miscarriages [
9,
35], implying an unascertained maternal cause for spontaneous abortion [
27]. However, other researchers have found no correlation between the number of previous miscarriages and chromosomal aberration in the aborted embryo [
36]. However, the study did not clarify whether chromosomal aberrations in embryos were inherited from balanced rearrangements in parental chromosome.
Parental chromosomal abnormality is an important genetic cause of spontaneous miscarriage. In this study, only one case of structural abnormality was found to be derived from maternal balanced chromosomal translocation. In accordance with previous studies, most miscarriages that occurred during early pregnancy were due to non-inherited chromosomal aneuploidy [
1,
2], suggesting the potential involvement of environmental factors in embryonic chromosomal teratogenesis and/or spontaneous abortion.
In this case-case study, the incidence of embryonic chromosomal aberrations showed a negative correlation with prenatal exposure to noise. The result indicated that prenatal exposure to noise may be related to spontaneous abortion in embryos with normal karyotype. Animal experiments have shown that noise may have a direct effect on developing animals by increasing the embryo absorption and decreasing live births per litter [
37,
38]. In addition, Noise may have an indirect impact by reducing the uteroplacental blood flow and increasing the release of catecholamines (one of the stress hormones), which may induce fetal hypoxia and abnormal embryogenesis [
39]. Till date, a few epidemiologic studies have investigated the association between noise exposure and spontaneous abortion. However, the quality of evidence of this association is very low [
40,
41].
Women with a history of exposure to paint showed higher incidence of fetal chromosomal aberrations, although, the association was not statistically significant; this may be attributable to the small sample size. Thus, exposure to paint may increase the risk of embryonic chromosomal aberrations. Volatile organic compounds (VOC) emitted from paint may cause chromosomal aberrations [
42,
43].
This was a small-scale study to estimate the risk factors of spontaneous abortions with and without embryonic chromosomal aberrations. In this study, we used both KL-BoBs and FISH technology to analyze the aborted POC tissues, which avoided the influence of cell culture failure and maternal cell contamination on the karyotype results [
3,
26]. This approach helped overcome the limitations of BoBs technique in analyzing polyploidy [
26,
44], and improved the detection rate and accuracy of the results. There are three main limitations of this study. First, this combined method is unable to detect chromosomal microdeletions or microduplications in regions not covered by the kit and balanced rearrangements [
24,
44]. Second, we did not perform next generation sequencing (NGS) and chromosomal microarray analysis (CMA) for copy number variations (CNVs) and/or single nucleotide variants (SNVs) in euploid miscarriages [
45‐
47]. This may have led to misclassification in the division of the case group and the control group. However, possible pathogenic CNVs were detected in ∼2% of miscarriages, and a large number of miscarriages had CNVs of unknown significance [
45,
47]. Mutations in genes involved in embryo implantation, angiogenesis, coagulation, immunological function response, and fetal growth, may contribute to or predispose to pregnancy loss [
45,
46,
48]. The clinically useful SNVs need to be validated by in vitro/in vivo functional tests. Third, some potential factors related to spontaneous abortion or embryonic chromosomal aberrations were not included in this study, such as assisted reproduction, thrombophilic disorders, immune dysfunction, and exposure to other environmental factors.
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