Main findings
Amongst the women who presented with threatened miscarriage and had POC available for analysis (n = 9), seven out of eight (88%) were found to have chromosomal aberrations in their POC. This percentage is consistent with studies showing that the presence of a major fetal chromosomal abnormality accounts for as many as 50% of all first-trimester pregnancy losses [1, 2, 10]. These women with an abnormal POC karyotype also had a lower mean serum progesterone level at 24.0 ± 23.3 nmol/L. This is consistent with studies that found progesterone levels to be lower in pregnancies with chromosomal abnormalities, particularly trisomies 16, 18, 21 and 22 [11, 12]. Progesterone plays an important role in maintaining the pregnancy and preventing miscarriage by preparing the uterus for implantation. It maintains decidualization of the endometrium to facilitate implantation [13]. Despite this, patients who were given progestogen supplementation (cases 1 to 5) still eventually miscarried, and chromosomal aberrations were found in all five POC. These results might lend a new perspective to the findings of the recently published randomized controlled trials of progestogens for the treatment of miscarriages. The Progesterone in Spontaneous Miscarriage (PRISM) trial showed that women with threatened miscarriage and subsequently randomized to receive vaginal progesterone or placebo, did not result in a higher incidence of live births in the treatment arm [14]. A recent randomised, double-blind controlled study conducted in Asia also concluded that oral progestogen given to women with first trimester threatened miscarriage did not reduce the risk of miscarriage or improve the eventual live birth rate [15]. The study found that oral progestogen treatment did not significantly reduce the rate of miscarriage between the placebo and treatment groups, which had a high pre-treatment progesterone level of 69 and 67 nmol/L respectively. The findings from our study could potentially account for the lack of response to progestogen treatment, especially when the underlying etiology for miscarriage is due to chromosomal aberrations. This suggests that progestogen supplementation is likely useful only in the context of low serum progesterone, particularly in the context of luteal phase deficiency. Low serum progesterone level had been associated with an increased risk of miscarriage [16]. Targeted treatment for this group of patients could potentially reduce the risk of miscarriage and increase live birth rates. This paves the way for future randomised controlled trials for women with low serum progesterone, who would most likely benefit from progestogen supplementation.
Of the seven POC with chromosomal aberrations, the most frequent was autosomal trisomy present in four (57%) POC; namely Case 1, 3, 5, and 7, which had a concomitant autosomal trisomy 22 with monosomy X. This result is consistent with numerous early miscarriages chromosomal studies, which similarly found that trisomy - whether for a single chromosome or multiple chromosomes (multiple trisomy), was the most common abnormality detected in POC of pregnancy loss [17]. It has been speculated that variations in nondisjunction and differential selection may be responsible [4]. Following trisomies, the second most frequent aberration detected were mosaic genome-wide UPD in all autosomes (case 2 and case 4) and Monosomy X (case 7 and case 9), each making up 29% of the genetically aberrant POC. The relatively high frequency of mosaic genome-wide UPD is surprising given that mosaic abnormalities do not typically constitute a high frequency of all chromosomal aberrations detected, in which mosaic abnormalities were found in 4.9% of all cases of chromosomal aberrations [18]. The genome-wide uniparental diploidies result primarily from a functional imbalance of virtually all parentally imprinted loci [19]. Monosomy X was found to constitute around 10% of chromosomal abnormalities detected in other POC studies in women who had an early spontaneous miscarriage, and it is the third most common abnormality after autosomal trisomy and polyploidy [20, 21, 22]. It is hypothesized that anaphase lag or other mitotic events are responsible for the loss of a sex chromosome in conceptions that started as diploid XX or XY [23]. The high incidence of genome-wide UPD mosaic and monosomy X in our study is likely to be contributed by the limited sample size.
Maternal age is a known risk factor for chromosomal aberrations and spontaneous miscarriages. An increase in chromosomal anomaly rate was found in women 35 years and older when compared with younger women [24]. Indeed, the mean maternal age of the patients who miscarried was older (32.7 ± 4.2 years). Regarding gestational age, patients with chromosomal aberrant POC took a shorter time of 0.74 ± 0.93 weeks to progress from threatened miscarriage to complete miscarriage as compared to case 7, which had a normal POC karyotype and progressed from threatened to complete miscarriage in 2.59 weeks. This could be explained by the strong correlation between chromosomal abnormality and severely disorganized fetal development resulting in developmental arrest and eventual abortion [1].
A normal karyotype and serum progesterone do not always result in a normal pregnancy (case 6). The etiology of an euploid miscarriage is likely multifactorial, with maternal causes, such as uterine structural anomalies, antiphospholipid syndrome, hereditary thrombophilia, other endocrine abnormalities and immune factors [20] or fetal causes, such as structural abnormalities, being commonly cited.
Implication for Clinical Practice
Serum progesterone is a good biomarker to triage women presenting with threatened miscarriage into a high or low risk group, thus guiding the treatment of high-risk patients with progestogens supplementation [16]. However, the success of the treatment will likely depend on the underlying etiology, which is not evident based on serum progesterone alone. Chromosomal aberrations are common in women who miscarried, and our study contributes to the existing literature in guiding the counselling for these women. This is especially important because knowing the possible irreversible etiologies of miscarriage can result in better closure and reduce the feelings of self-blame. This would provide these affected women with reassurance and confidence as she prepares for her subsequent pregnancy [25].
There is limited data on the risk of miscarriage in subsequent pregnancies, after a previous spontaneous miscarriage. Early spontaneous miscarriages are thought to occur on a random basis and most patients are likely to have a successful pregnancy after a first miscarriage [26–28]. Although some studies that suggested a risk of recurrence of both autosomal and sex chromosome fetal aneuploidy in women with a previous spontaneous miscarriage of aneuploid conceptions [29–32], there is insufficient evidence to routinely screen all patients after one miscarriage. However, POC analysis may be done for patients who experience three or more miscarriages, with subsequent parental karyotype being tested if inheritable genetic aberrations are found, such as balanced translocations [33, 34].