The word "coronavirus" alludes to the spikes on the virus's surface that resemble crowns. Wuhan coronavirus and 2019 new coronavirus are the names given by Chinese experts to the novel virus (2019-nCov). The COVID-19 and SARS-CoV-2 designations for the disease and virus, respectively, were granted by the International Committee on Taxonomy of Viruses (ICTV) [1]. Coronaviruses are members of the Nidovirales order and belong to the family Coronaviridae.

There are four subgroups of the coronavirus family: Alpha (a), beta (b), gamma (c) and delta (d). According to reports, this coronavirus belongs to the beta group [1]. The diameter of coronaviruses ranges from 65 to 125 nm and their nucleic materials are positive single-stranded RNAs with lengths of 26 to 32 kb [1]. The genome of a coronavirus consists of 30.000 nucleotides. It encodes four structural proteins: Spike, Membrane (M) protein, Nucleocapsid (N) protein, Envelope (E) protein, various nonstructural proteins (nsp) and the (S) protein [2]. Close contact with an infected individual who has been exposed to their coughing, sneezing, respiratory droplets or aerosols allows the virus to transfer from one person to another [1]. Despite not being a sexually transmitted disease, COVID-19 may nevertheless spread by intimate physical contact, such as kissing and fecal-oral routes [3]. The two most frequent symptoms of COVID-19 are fever (88 percent  of  cases)  and  dry  cough  (49%  of  cases) (68 %). Pathogenesis has three stages: Stage 2 is when the virus has disseminated and is causing moderate sickness as it moves down the vector airways and descends the respiratory system. Stage 1 is the asymptomatic period when the virus adheres to epithelial cells in the nasal cavity and begins multiplying. Stage 3: The virus now infects type II alveolar cells in the lung, leading to a lack of oxygen and progression to ARDS [4]. A woman's womb or uterus is where one or more children grow during pregnancy, commonly referred to as gestation. Contrary to popular belief, assisted reproductive technology operations can result in pregnancy. Usually, 40 weeks after the start of the last menstruation, a child is born (LMP). A fetus is used until delivery to describe the growing child during the first eight weeks after conception (ten weeks gestational age). Each of the three trimesters of pregnancy lasts for around three months. Conception occurs during the first trimester when the sperm fertilizes the egg. Movement of the fetus may be felt towards the middle of the second trimester. The third trimester is referred to either at the start of week 28 or week 29. It continues till childbirth [5].

Several studies have demonstrated a link between respiratory virus infections during pregnancy and issues including low birth weight and premature birth. Furthermore, a high fever at the beginning of pregnancy may make several birth abnormalities more likely [6]. The chance of miscarriage was somewhat increased in women with the SARS-CoV infection but only in those who were also experiencing additional problems. No research has yet shown that SARS-CoV-2 infection during pregnancy increases the risk of miscarriage [7]. So, this review summarized the study's effects of Covid-19 infections during pregnancy and the potential risks of Covid-19 infections among pregnant women and consequent fetal transmission.

Classification of Coronavirus

The SARS-CoV-2 belongs to Coronaviridae, sub-family Orthocoronavirinae and order Nidovirales. The genus Alphacoronavirus, Betacoronavirus, Gammacoronavirus and Deltacoronavirus are further classified into these four families [8]. While the genera Alphacoronavirus and Betacoronavirus are descended from bats, Gammacoronavirus and Deltacoronavirus have their origins in avian and swine gene pools [9]. According to reports, SARS-CoV-2 is a coronavirus in the beta group [1].

Structure of Coronavirus 2019 (SARS-CoV-2)

The diameter of coronaviruses ranges from 65 to 125 nm and their nucleic materials are positive single-stranded RNAs with lengths of 26 to 32 kb. (Figure 1) [1]. A coronavirus has a genome made up of 30.000 nucleotides. It produces various nonstructural proteins as well as membrane, nucleocapsid (N) protein, four structural proteins (M), protein, spike (S) protein and envelope (E) protein (nsp) (Figure 1) [2].

Figure 1: Structure of SARS-CoV-2

 The ORF1 downstream regions of all coronaviruses contain particular genes that code for proteins necessary for viral replication, nucleocapsid development and spike generation. Over 80% similarity between the SARS-CoV-2 and the preceding human coronavirus has been identified in its genome (SARS-like bat CoV). The pp1ab protein and 15 nsps are encoded by the orf1ab gene, which is the biggest gene in SARS-CoV-2. The pp1a protein, which likewise has 10 nsps, is encoded by the orf1a gene [1].

NSPs (Non-Structural Proteins) and Accessory Proteins

The SARS-CoV-2 genome has 8 auxiliary proteins, nsps 1 through 10, nsp 12 through 16 and 15 nsps, in addition to the essential structural proteins (3a, 3b, p6, 7a, 7b, 8b, 9b and orf 14). To the reproduction of viruses, each of these proteins has a distinct contribution. Because it lacks the longer 8b and shorter 3b proteins, as well as the 8a protein  and  longer  8b,  SARS-CoV-2  is  different  from SARS-CoV.  The  auxiliary  proteins  p6  and  8b,  nsp7, nsp13, envelope and matrix, in contrast to coronaviruses, have not been discovered to contain any amino acid alterations [1].

Transmission of Coronavirus 2019 (SARS-CoV-2)

It was assumed that the SARS-CoV-2 pneumonia patients in China either visited a seafood market where live animals were sold or consumed contaminated animals or birds [1]. A zoonotic (animal-to-human) transmission may have occurred here (Figure 2) [10]. Later studies, however, revealed that more cases of the virus's spread occurred without any evidence linking them to the fish market. Because these results indicated that it might transmit from person to person, this virus was reported in more than 100 different nations. Close contact with an infected individual exposed to respiratory droplets, aerosols, coughing or sneezing allows the virus to pass from one person to another. By inhaling these aerosols via the mouth or nose, a person might expose their lungs to them [1]. Aerosols are smaller, longer-lasting droplets that can cling to the air. According to results obtained in previous studies, the virus can stay in the air for up to three hours [11]. Less often, infected droplets that fall on floors or other surfaces may still be contagious if someone touches them with unwashed hands before touching their eyes, nose or mouth. Less often, infected droplets that fall on floors or other surfaces may still be contagious if someone touches them with unwashed hands before touching their eyes, nose or mouth. The virus can be found on copper surfaces for up to four hours, cardboard for up to one day, plastic (polypropylene) and stainless steel (AISI 304) for up to three days [12]. Large amounts of the virus are found in both sputum and saliva [13]. Despite not being a sexually transmitted disease, COVID-19 can spread by direct contact, including kissing, close physical contact and fecal-oral routes [3]. The virus may be present in breast milk but so far it has not been detected that it has reached the newborn [14].

Figure 2: Transmission of SARS-CoV-2

Replication Cycle

Angiotensin-converting enzyme 2 (ACE2), the SARS-human  CoV's  cell   receptor [15],   is   located  in  the  lower respiratory tract and controls both interspecies and human-to-human transmission [16]. The SARS-CoV-2 employs the same cellular entry receptor, ACE2, as the SARS-CoV, according to research by Zhou et al. [17], isolated from the bronchoalveolar fluid (BALF) of a COVID-19 patient.

• Cell entry: The viral protein binds with the host cell receptors corresponding to the onset of infection (ACE2). The protein attached to the receptors is activated after attachment, by division by the protease of the host cell. Depending on the protease of the host cell, the virus may enter the host cell through internal poisoning or membrane fusion [18]
• Genome  translation:  When  the  viral  particle enters  the  host  cell,  it  loses  its  coating,  enabling its genome to reach the cell's cytoplasm. The coronavirus  RNA  genome  can  act  as  a  messenger RNA and be directly translated by the host cell's ribosomes  since  it  has   a   3 polyadenylated   tail and a 5 methylated cap [30]. PLpro (nsp3) and 3CLpro (nsp5), two of the polyproteins' proteases, break the polyproteins at various locations. 16 nonstructural proteins   result   from   the   cleavage   of  the polyprotein pp1ab (nsp1 to nsp16). Exoribonuclease (nsp14),  RNA  helicase  (nsp13)  and  RNA-dependent RNA  polymerase  (nsp12)  are  examples  of replication   proteins   that   are   produced [19]

• Replication: When many of the nonstructural proteins join together, they form a multi-protein replicase-transcriptase complex (Figure 3). The main protein involved in replicase-transcription is RNA-dependent RNA polymerase (RdRp). It participates actively in the transcription and replication of RNA from an RNA strand. The complex's other nonstructural proteins take role in transcription and replication as well

Figure 3: Replicase-Transcriptase

A nonstructural protein called exoribonuclease increases replication fidelity by carrying out a proofreading function that RNA-dependent RNA polymerase is unable to carry out [20].

• Replication: The complex's ability to reproduce the viral DNA is one of its primary tasks. RdRp directly mediates the production of negative-sense genomic RNA from positive-sense genomic RNA. And then, from negative genetic RNA, positive RNA is replicated [19]

• Copying: Another important function of the compound is the transcription of viral DNA. RDRP directly mediates the synthesis of negative RNA submolecules   from    RNA    from    positive    RNANext, these negative RNA molecules were translated into positive mRNAs [19]. The "nested set" of subgenomic mRNAs (Figure 4) has a shared 5'-head and a largely duplicated 3' end

• Recombination: When there are two viral genomes in the infected cell, replication synthesis also leads to genetic  recombination [21].  Although  the  exact mode of recombination in coronaviruses is unknown, mold switching is likely to occur during genome replication [22]

• Collection and release: The progeny viruses' genomes are created from the replicated positive-sense genomic RNA. The mRNAs represent the final third of the viral genome, which follows the initial overlapped reading frame. The host's ribosomes use these mRNAs to translate them into a variety of auxiliary proteins and structural proteins [19]. RNA translation occurs in the endoplasmic network where the secretory pathway allows viral structural proteins S, E and M to reach the intermediate Golgi compartment and the majority of the protein reactions needed to capture viruses in nucleocapsid after they are connected to them by proteins M. (Figure 5) [19]

• The method of development of the disease: The incubation period of the virus is estimated to be between two and 14 days and by 95% the incubation period falls in about 5 days [38]. Covid-19 can be divided into three stages, depending on the most likely infected cells, each of which is associated with a different clinical stage of the disease [4]

Figure 4: Transcription

Figure 5: Replication Cycle of SARS-CoV-2

Stage 1: (1-2 Days of Infection) Symptomatic Condition

SARS-CoV-2 is more likely to bind to nasal epithelial cells and begin to multiply in them after inhalation, as the main receptor of SARS-CoV2 and SARS-CoV is ACE2 [17]. At this stage, although the immune response is very mild, the virus spreads and the virus can be identified using nasal swabs Although these people may have a low viral load, it is contagious [23].

Stage 2: Upper Airway and Conducting Airway Response (Next Few Days)

At this stage when the virus spreads and migrates along the vector airways of the respiratory tract, the immune response is stronger and Covid-19 is a clinically noticeable disease [23]. As in this stage the disease is mild and falls short primarily in the upper airways and about 80% of the infected patients are treated and these patients may receive treatment while being followed up at home[4]. 

Stage 3: Hypoxia, Ground-glass Opacification and Progression to ARDS

However, 20% of those infected may suffer from this stage, some patients may suffer from severe illness and the mortality rate at this stage is expected to be around 2% and this varies greatly with age [4,23]. The lung's gas exchange   units   are   now   infected   with   the   virus    via alveolar type II cells [24]. When type II cells are exposed to SARS-CoV, many virus particles are produced and the cells eventually die (Figure 6) [25]. At this stage, the  result is also likely to be self-replicating pulmonary toxins when virus particles infect type II cells in nearby units. Morgibly, COVID-19 and SARS cause widespread alveolar damage along with a few large polynuclear cells and fibrin-rich hyalene membranes [26]. Compared to other ARDS, aberrant wound healing may cause more severe scarring and fibrosis [39]. Finally, at-home management of COVID-19 contained in the conducting airways should be gentle and symptomatic. However, COVID-19 must be carefully monitored and cared for to the best of our abilities now that it has spread to the lung's gas exchange units.

Figure 6: Human alveolar cells of type II with SARS-COV infection, SARS-COV was introduced into isolated type II cells in the laboratory

Specimen Collection and Laboratory Diagnosis 

A nasopharyngeal and/or pharyngeal swab should be taken for initial examination and early detection of infection. High virus loads were seen in upper respiratory tract samples 3-5 days after symptoms began. Swabs should contain handles of synthetic nylon and non-toxic synthetic fibers, such as polyester. The World Health Organization proposes that the swab be taken as a mixture of the nasopharynx and it is important to keep in mind that the collection of swabs through the mouth and nose separately or simultaneously is placed in a single section of the viral transmission medium. Swabs should be collected and placed in vials containing a viral transport medium (VTM), transported to the laboratory, within 24 to 72 hours, preferably in a refrigerator (4-8° C). If there is a delay in shipping, the samples should be kept at -70°C [27]. Lower respiratory samples of sputum and bronchial aerobic are used for late detection and monitoring of people with severe Covid-19 pneumonia as there is a study that showed that the greatest SARS-COV-2 RNA is produced in the bronchi and large viral loads have also been detected in the feces of some Covid-19 pneumonia patients late in their clinical illness, therefore, rectal swabs can be suggested instead of direct respiratory samples for diagnosis in advanced COVID-19 patients [28].

Reverse Transcription Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) produces enough material for analysis by amplifying (repeating) a small, precisely defined area of DNA, thanks to the use of certain chemicals, DNA can be recovered from test materials. Reverse transcription of RNA results in the production of DNA. Reverse transcription polymerase chain reaction (RT-PCR) is a technique for obtaining DNA through reverse transcription and amplifying it using PCR to produce enough for analysis. SARS-CoV-2, which only includes RNA, may thus be found using RT-PCR. RT-PCR procedures typically take a few hours. Automation, better throughput and more dependable instrumentation are some benefits of real-time PCR (qPCR). It has evolved into the favored approach. The sensitivity range of RT-PCR for clinical samples is 63% for nasal swabs, 32% for pharyngeal swabs, 48% for feces, 72-75% for sputum and 93-95% for alveolar bronchial lavage [29].

Isothermal Amplification Assays

The genome of the virus has also been amplified by isothermal DNA amplification experiments, since they do not involve frequent heating and cooling cycles, they are faster than PCR, as these tests are often used to identify DNA that is read by specialized equipment. To carry out the detection, CRISPR technology was developed to edit genes, as the CRISPR enzyme is linked to sequencing, it colors a paper stripe in point-of-care places, the researchers expect the final test to be inexpensive and simple and the test amplifies the RNA instead of converting it first into DNA, as in RT-PCR [30].

Antigen Tests

A pathogen's component that causes an immune reaction is called an antigen. Proteins from the viral surface are sought by antigen testing. These are frequently proteins from the spikes on the surface of a coronavirus [31]. Compared to PCR testing for the virus, SARS-CoV-2 antigens can be identified more quickly but with lower sensitivity before COVID-19 symptoms (as soon as SARS-CoV-2 viral particles) [32].

Antibody Tests

Antibodies   are   created   by   the   body   in   response   to viral  infection  to  aid  in   the   virus's  neutralization.  Such antibodies can be found through blood testing (serology tests). Antibody testing may determine how much of a population has previously had the illness, which can subsequently be used to determine how many people die from it [33]. IgM and IgG antibodies are the most noteworthy ones. Although the levels of IgM antibodies during the duration of the illness and after that are not well described, they are often detectable several days after the first infection. IgG antibodies typically peak 28 days after infection and are usually detected 10-14 days after infection. IgM is often discovered 5 days after the beginning of symptoms, whereas IgG is typically detected 14 days after the onset of symptoms. After two or three months, there is a considerable drop in IgG levels [34]. The average sensitivity of antigen testing is 56.8% and the average specificity is 99.5%. However, there is a huge difference between test results from different firm brands regarding sensitivity (ranging from 0 to 94%) [35].

Imaging

Bilateral multilobar ground-glass opacities with a posterior or peripheral distribution are often seen in the first CT results. With CT rather than RT-PCR, COVID-19 may be detected with more accuracy. With time, the condition may progress to subpleural dominance, crazy paving and consolidation. It is not advised to diagnose COVID-19 with a chest CT scan or a chest x-ray. In COVID-19, radiologic results are not specific [36].

Effects of Covid-19 on Pregnancy

However, it was noticed that miscarriages were more common in women with the SARS-CoV infection but only in those who were also experiencing additional problems. [7]. No research has yet shown that SARS-CoV-2 infection during pregnancy increases the risk of miscarriage. Despite the paucity of available data, there is no indication that pregnant women are more vulnerable to coronavirus infection than non-pregnant individuals (SARS or MERS). Men have been afflicted at a higher rate than women in this epidemic of new coronavirus infection thus far [37]. This gender disparity may result from variations in reporting, susceptibility, exposure or the identification and diagnosis of infections. There is no evidence to indicate that COVID-19 susceptibility increases during pregnancy. Clinical results during pregnancy can range from no symptoms to severe illness and death, according to earlier studies on SARS and MERS. More than 80% of hospitalized COVID-19 patients come with fever and cough, the disease's most prevalent symptoms [38].

Pregnant women's health must be considered throughout this coronavirus pandemic, which is evolving quickly. For pregnant women, it is crucial to offer the key interventions they need. According to the World Health Organization (WHO), there does not appear to be a difference in the likelihood that pregnant and non-pregnant women of reproductive age would experience clinical symptoms [39].

The latter is likewise not at a larger risk of contracting a serious illness. Although some patients may be asymptomatic [40-44], patients most frequently come with moderate infection-related symptoms such as fever, cough, exhaustion and shortness of breath. In their retrospective assessment, Liu et al. [45] Comparing 59 individuals, including pregnant and non-pregnant people, regarding the appearance of clinical characteristics of SARS-COV-2, this study found no clear differences between different groups. Immune systems are changed due to physiological changes experienced by pregnant individuals. They may respond to COVID-19 infection similarly to how they would respond to any other viral infection since this does not necessarily make them more vulnerable to viral infection. However, despite the minimal likelihood, people may encounter severe symptoms due to the modified immune system. One research found that pregnancy did not affect the symptoms of COVID-19-related pneumonia or the results of a CT scan [40].

Considering the recent COVID-19, there isn't enough information to conclude the virus's potential effects on a developing fetus. It may be feasible to determine the result of the COVID-19 viral process by comparing it to previous coronavirus infections, such as SARS and MERS [46,47]. According to three trials, there were no maternal problems, such as premature labor and postpartum COVID-19 infection [40,41]. However, several additional trials have shown problems for both the mother and the fetus, such as premature birth, respiratory distress, fetal distress and PROM [44].

According to a report published in Iran, maternal deaths alone and intrauterine death have been directly linked  to  Covid-19  infection  during  the  last  trimester of   pregnancy [46].   Of   the   total   of   nine   pregnant women  from  Iran  diagnosed  with  severe  Covid-19 disease in the second or third trimester, 7 of the nine patients perished and one is still very sick and dependent on a ventilator and one of them has fully recovered after a long hospital stay [48]. Additionally, Baud et al. [49] described a case of miscarriage in a pregnant woman with COVID-19 during the second trimester of the pregnancy that looked to be connected to placental infection with SARS-CoV-2. The Royal College of Obstetricians and Gynecologists (RCOG) believes that fresh research suggests  that  vertical  transmission  from  the  mother  to her unborn child may be conceivable. By monitoring the fetal IgM blood level, a few recent investigations have indicated the likelihood of in-utero COVID-19 transmission [50-52].

Since the IGM antibody cannot cross the placenta, it is assumed that this may represent the immune response of the developing fetus to infection. A baby with Covid-19 infection was found in one search 36 hours after birth where SARS-COV-2 nucleic acid was not found in the placenta or umbilical cord blood of this patient, according to further research into the possibility of intrauterine transmission and provides reports of previous cases from China showing data indicating no association in intrauterine transmission, According to this research, samples of amniotic fluid of infected women, umbilical cord blood, vaginal swabs, breast milk and throat swabs for   newborns   were   all   negative   for    COVID-19,   which means that all infants have been screened so far [53,54]. However, Penfield et al. [55] reported a study in which SARS-CoV-2 was detected in 3 of the 11 placental or membrane swabs taken after childbirth from women with moderate to severe COVID-19 disease at the time of birth, this study is the first to demonstrate that SARS-CoV-2 RNA can be found in placental or membrane samples. Their findings on the possibility of exposure to the virus during childbirth were highlighted despite the lack of clinical indications of vertical transmission, so caesarean section is the form of delivery that is recommended to reduce the period of exposure to these tissues and reduce the possibility of vertical transmission. Only a few recorded cases, when all the information is taken into account, allow us to determine whether SARS-CoV-2 may travel inside the uterus vertically. As a result, it is not yet possible to predict any conclusion with certainty. According to the RCOG, there is not enough information to prove a link between the COVID-19 risk of abortion and the possibility that the virus is teratogenic so more research needs to be done on the likelihood of vertical transmission as the results so far are contradictory. When there is evidence that the mother has COVID-19, the recorded fetal outcomes include respiratory distress in the womb, PROM and premature birth [41,42,47].

On the impact of COVID-19 on pregnancy, fetus and baby are currently limited to a few small case chains due to the novelty of COVID-19. While pregnant women perform better than the group aged 80 years and older, they are nevertheless exposed to premature births at a rate of three times the rate and twice the incidence of pregnant women. Available data do not sufficiently support the status of vertical transmission of SARS-CoV-2 infection to the unborn child. Since there is no reliable information on how COVID-19 affects newborns, care should be used in future research and possibly infection control in newborns delivered to mothers with COVID-19.

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