Recurrent the causes remain unknown, which is called

pregnancy loss (RPL) is one of the most important abnormalities during
pregnancy which, in its definition should be considered two features: 1- Occurrence
of at least two successive miscarriages in previous pregnancies 2- These miscarriages
occurred before the 20th week of gestation. The prevalence of RPL in pregnant
women is about 1-5%. Several causes have been reported for this disorder, of
which the most important are: Genetic anomalies, immune and biochemical
disorders, thrombophilia, infections, uterine anatomical disorders, and
lifestyle. However, more than 50% of cases, the causes remain unknown, which is
called unexplained recurrent pregnancy loss (uRPL). Cell free DNA and RNA in
maternal plasma can be important as non-invasive biomarkers in controlling
pregnancy and diagnosing pregnancy-related disorders and one of these RNAs is non-coding
RNAs. MicroRNAs (miRNAs), as a type of small non-coding RNAs, are involved in the
process of inhibiting the expression of genes by two ways: blocking translation
and breaking of mRNA. The miRNAs are derived from a miRNA precursor, which,
after processing through molecule complexes of Dorsha (in nucleus) and Dicer
(in cytoplasm), situated in a RNA-induced silencing complex (RISC). The detection
of target mRNAs is carried out by this complex. Many studies have shown the
involvement of miRNAs in pregnancy and RPL. These can play several roles in
this reproductive disorder, as discussed below: by reducing the expression of
genes, miRNAs can cause abortion, for example, by reducing the expression of
genes, miRNAs can cause miscarriage. For example, miR-133a is upregulated in
patients with recurrent miscarriage, and can lead to abortion through
decreasing the HLA-G expression at the protein level. Also, in specific
populations, some polymorphisms of miRNAs have different expressions, so this
can increase the risk of RPL in those populations. In 2012, a study showed that
in patients with spontaneous abortion, microRNA polymorphisms (miR-146aC> G,
miR-149T> C, miR-196a2T> C and miR-499A> G) are considered as a risk factors
of RPL. On the other hand, circulating miRNAs can serve as a biomarker in the
disorder, as shown in the study by Qin et al., that five miRNAs can be as diagnostic
markers for RPL.