Discussion
The cause of spontaneous abortion is generally attributed to two sources, namely seed problems and environmental problems. Seed problems are often considered to be abnormal parental chromosomes or abnormal fetal chromosomes. The results of this study showed that the incidence of chromosomal abnormalities in couples with recurrent miscarriage was 3.74%(Figure1). The present results consistent with previous studies have shown that the incidence of chromosomal abnormalities in the general population is less than 1%[15, 16] and RM population is 2%-5%[17, 18], indicating that parental chromosomal abnormalities rate increased assuredly in the miscarriage couples.
Balanced translocation was the most common type, accounting for 38.02% and consistent with other findings[11]. A meta-analysis from Zouhair reported that frequency of chromosomal abnormalities in couples with RM was 5.16% and the most common reciprocal translocation accounts 48.4% in the worldwide literature review[12]. The balanced translocations and inversions will not affect the parents themselves in phenotype, but their unbalanced gametes during meiosis may indeed be part of the cause of miscarriage. Similarly, Robertsonian translocation of parental chromosomes can also cause miscarriage, birth defects or mental retardation of offspring[19]. However, all these studies could not demonstrate the explicit causality between aberrant chromosome and abortions.
Additionally, the LBR of RM carriers in our reproductive center have reached more than 70%, indicating that the proportion of miscarriage caused by chromosomal abnormalities in RM couples was very slight. A retrospective study from Howard et al. concluded that no statistically significant was found in the LBR between RM couples with chromosomal abnormality (45.2%, 33/73) and the normal couples was (55.3%, 325/588), regardless of number of miscarriages and rearrangement types of chromosomal abnormalities [20]. In Goddijn’s study the screening results of 1324 RM couples showed that all the 41 couples with abnormal structure chromosomes did not yield an unbalanced fetal chromosome pattern [21]. It is also consistent with Franssen’s study, the LBR of RM carriers was equivalent to the normal couples after six accumulated gestations, and had no relevance with the type of abnormal chromosome (83% vs. 84%)[22]. However, Sugiura’s study showed pregnancy prognosis was worsened with either maternal or paternal reciprocal translocations than normal couples (63% vs. 78.7% of LBR)[23]. Pregnancy outcomes for RM couples with chromosomal abnormalities were still very satisfactory generally, although the decrease in the live birth rate may not have been detected due to insufficient sample size in our study.
Preimplantation genetic testing (PGD) has been proposed as a controversial method in the worldwide for selecting normal chromosome embryos in the IVF to lower risk of miscarriage for patients with unexplained RM and balanced translocations carriers. However, well-designed trials comparing EM (expectant management) to PGD have not been performed. Several previous cases indicated benefits of PGD including fewer miscarriages and shorter time to successful pregnancy without taking into account the emotional and financial cost of a failed or canceled cycle. More recent reports suggested clinical outcomes including pregnancy rate, live birth rate (53% vs. 67%) and clinical miscarriage rate were similar between PGD and EM among recurrent miscarriage patients[24]. Even in the parental carriers of structural chromosomal rearrangement and history of RM, no significant difference with regards to reproductive outcomes such as miscarriage rate, time to live birth, or live birth rate was observed between couples who pursued PGD compared with EM [25, 26]. These data combined with our results allow us to reflect on the actual benefits of PGD to these patients, so clinicians can be more cautious when making an alternative of PGD in clinical work. Natural conception is also recommended as a good alternative for these aberrant chromosomal carriers.
50%-60% of spontaneously aborted product of conception have been detected with chromosomal abnormality [27]. The abnormal chromosomes of the fetus are derived from the parental abnormal chromosomes or produced in the process of gamete meiosis and mitosis of the fertilized egg by mistake randomly. The types of fetus abnormality were often mainly manifested as trisomies of chromosome 13, 18, 21 and X monosomy (45, X) [28], but not consisted with the translocation chromosomes of the parents showed as in the Figure 3. According to Howard Carp’s study, parental karyotyping was not particularly predictive of a subsequent miscarriage, 43.5% of abortus from parental carriers were euploidic and the parental aberration was passed on to the abortus in only 10% of cases[29]. The phenotypes are inconsistent that parental karyotyping prefers balanced translocations (No. 8, 2, 6) and inversions (No. 9, 1, 6) rather than the more common numerical aberrations such as trisomies (No. 13, 18, 21) and polyploidy in fetus. Most aberrant chromosomes in the fetus are generated randomly and only a small percent derives from their parents.
One of the most important results in our data is the influence of parental chromosomes on live birth rate (LBR). In our study, the LBR of both carriers and non-carriers can reach about 70% without relationship of gender, female age, chromosome abnormal type, abortion times and other pathological factors. Amounts of non-genetic pathological causes related to endocrine, infection, immune and nutrition were detected not only in aberrant chromosomal carriers but also in non-carriers, while these factors have a strong impact on the pregnancy outcomes. After effective treatments such as anticoagulation and immunotherapy, the LBR of re-pregnancy after two recurrent miscarriages has reached more than 70% internationally. A prospective study showed that closely following management and treatment of other high-risk factors can increase the LBR of RM couples with chromosomal abnormalities from 25% to 70%[30] or from 20% to 71% without the addition of assisted reproductive technology[11]. The differences of LBR in RM carriers in previous reports may due to the different management of non-genetic pathological factors that are usually more important in fetal survival.
From these data, we strongly recommend that RM carriers should still undergo comprehensive and systemic etiological screening. Therefore, it is necessary to actively deal with other causes of miscarriage in order to improve the chances of successful pregnancy for RM patients with chromosomal abnormalities. In order to improve the live birth rate, our treatment included surgical correction of the anatomically abnormal uterus, male lymphocyte treatment, anticoagulation aimed at anti-phospholipid antibody and immunosuppressive therapy were strongly recommended besides chromosome abnormality in our opinions. The formation frequency of abnormal gametes theoretically is not equal to the birth rate of abnormal babies in practice, we still recommend that the chromosome test or next-generation sequencing analysis of the amniotic fluid through puncture should be performed around 18 weeks of gestation for the natural pregnancy of RM patients with chromosomal abnormalities, although the deletion or duplication of smaller fragments still cannot be detected.
The present study did not detect the karyotype of aborted fetuses and not achieve complete amniocentesis results from the pregnancy carriers. So we could not assess the impact of fetal chromosomal problems came from parents. In the study of embryo chromosome analysis of abortion tissue, trisomy and polyploidy are the majority which account for 65% and 17% respectively, a considerable proportion of fetus with aberrations include trisomy, structural abnormality and low-frequency mosaic could survive after birth[28]. The phenotypes are inconsistent that parental karyotyping prefers balanced translocations and inversions rather than the more common numerical aberrations such as trisomy and polyploidy in fetus. In addition, G-banding karyotype analysis used in this study can only detect a part of patients with abnormal numerical and structural chromosomes. Conventional karyotype analysis identifies balanced and unbalanced chromosomal rearrangements and copy number variants (CNVs) to a∼5 Mb resolution. Due to the limitations of the detection method itself, it could not exclude some other types of genes or chromosome abnormalities related to miscarriage problems, such as deletion, insertion, duplication and point mutation of some gene fragments. In 2019, Chen et al. used low pass genome sequencing (GS) to detect the chromosomes of RM couples with abnormalities rate increased to 11.7% compared to traditional karyotyping with 5.7%. However, inversions and copy-number variants detected by GS additionally had not been confirmed to directly related with miscarriage. 10 carriers observed in follow-up observations and five of them miscarried again (miscarriage rate of 50%). The small sample size did not indicate that the risk of miscarriage of abnormal chromosome couples was higher than that of couples with normal chromosomes[31].
Finally, the lack of samples even in this 11-year study and other combined known and unknown non-genetic factors are shortcomings in the present data. The etiology of recurrent miscarriage is complicated and there are many controversies in the treatment. The coexistence of these other pathological factors and chromosomal abnormalities makes the results confused and controversial.