The coronavirus responsible for SARS-Cov2 infection is an encapsulated RNA virus, member of the Nidovirales order. Its transmission is mainly through Flügge's droplets, but other ways of transmitting the infection have been demonstrated, the presence of viral RNA was detected in feces, sperm, hematogenous, mucous tissue.¹

Our paper is a review of the literature that addresses the hypothesis of vertical transmission of infection with the new coronavirus,hypothesis difficult to prove given the limited and constantly changing data.

In the case of a target group, such as pregnant women, with a modified immune status, the evolution of the infection is burdened by complications, with an increased risk of morbidity and mortality.

Vertical transmission of the infection can occur in the uterine life, in which case we are talking about a congenital infection, in the peripartum or in the immediate postpartum, each of them having a different clinical and biological resonance.

Once entered in the body, the virus replicates and settles in target organs, such as the lungs, heart, gastrointestinal tract, but given its extremely low molecular weight, it could also cross the feto-placental barrier.²

Review

Our paper is a review of some case studies from February 2020 up to present, of retrospective type, presented in the medical literature, concerning SARS-CoV-2 infection in pregnant women, as well as the hypothesis of its vertical transmission.

Possible transmission mechanisms were considered: an ascending infection through the cervix, in the case of pregnant women with ruptured membranes, because its presence has been demonstrated in the vaginal mucosa, so that virions can ascend to the uterine cavity, a situation that can be complicated by chorioamnionitis, hematogenous infection, fecal contamination, breastfeeding the newborn by a positive mother.

The placental immune response and tropism of certain viruses affects the course of pregnancy and causes an increased susceptibility to infections.

Although progesterone is considered a natural immunosuppressant, the concept of partially suppressed immune status in pregnancy is misleading. During normal pregnancy, the decidua contains an increased number of immune cells, such as macrophages, natural killer cells and regulatory T lymphocytes, which accumulate in it in the first trimester of pregnancy and invade the trophoblast. Thus, the presence of these cells at the site of implantation of the gestational sac is not associated with an immune response similar to the immune response from the contact with a non-self, but, on the contrary, the immune system is active, functional and fully controlled, in order to maintain the pregnancy and protect both the embryo and the mother.³

It has been found that there is a specific protein that allows the virus to infect human cells, called the angiotensin 2 conversion enzyme or ACE2 receptor, which is attached to the cell membranes of cells in the lungs, heart, blood vessels, kidneys, liver, gastrointestinal tract.

This enzyme is essential in the renin-angiotensin-aldosterone system (RAAS), helping to modulate the activity of angiotensin II, which is known to be responsible for regulating processes such as blood pressure, wound healing and inflammation. When the SARS-CoV2 virus binds to the ACE2 receptor, it prevents ACE2 from fulfilling its function of regulating angiotensin II signaling, thus increasing inflammation and cellular apoptosis, predominantly in the lungs and heart.

However, it is important to note that placental ACE2 receptor expression in the first trimester of pregnancy is much lower compared to the third trimester, most likely, this is why teratogenicity is reduced.⁴

In our paper we analyzed 70 newborns from SARS-CoV-2 positive mothers, the data were selected from 4 retrospective studies, 4 case studies and one cohort study.

Most pregnancies were completed to full-term, but premature birth was reported in 5 studies. The preferred method of birth was by Cesarean section in 57 of 70 cases, while vaginal birth was reported in 13 cases.

None of the newborns tested positive for COVID-19.

The majority of newborns who received nasopharyngeal exudate for coronavirus detection by RT-PCR were negative (65 newborns), but 4 patients from 2 different studies (Lan Dong, Huijun Chen) showed signs of early-onset infection in the first place, from the first two days of life, with positive RT-PCR examination, in which vertical transmission could not be excluded.

A newborn with negative nasopharyngeal exudate showed Ig M and Ig G positive serology (Dong et al.) and was considered potentially infected in utero.⁵

In 6 studies (Chen W et al., Zhang et al., Wang et al., Liu et al., Wang X et al., Dong et al.) in which different methods of investigation were used, the absence of vertical transmission of SARS-CoV-2 was reported, in 3 other studies (Wang S, Ling Zeng, Wang X) vertical transmission was impossible to rule out.⁵

All newborns were tested by RT-PCR techniques with nasopharyngeal exudate. As additional test methods the following were examined: amniotic fluid in 2 studies (Huijun Chen, Wang X) with negative result, placental blood was examined in 4 studies (Huijun Chen, Wang X, Liu W, Wang S), and breast milk was analyzed in 3 studies (Lan Dong, Huijun Chen, Wang X), all with negative results.

A recent study from March-April 2020 by Dr. Claudio Fenizia of the University of Milan, performed on a group of 31 pregnant women in the third trimester of pregnancy showed that vertical infection from mother to fetus is possible.Specimens from breast milk, vagina, placenta, amniotic fluid, umbilical cord blood were examined that showed the presence of the virus as well as antibodies in a vaginal swab, in a sample of breast milk and one of the placenta, as well as in nine samples of blood taken from the umbilical cord.⁶

Out of 31 newborns, only 2 tested positive for COVID-19, with negative test results occurring relatively quickly, suggesting that fetal infection is rare.⁶

The study gives weight to the evidences that shows the possibility of vertical transmission of the virus.

Another study considered important to support the hypothesis of vertical transmission was conducted in China, Zhongnan region, Wuhan University Hospital, at the beginning of the pandemic, where 9 cases of pregnant women testing positive for SARS-CoV2 infection were analyzed, none of which resulted in maternal-fetal death. Although the vertical transmission of the new coronavirus infection could not be clearly demonstrated, there was an increased risk of premature rupture of membranes, premature birth, fetal tachycardia, and acute fetal distress if the infection occurred in the third trimester.⁷ Of the nine births, four occurred prematurely, less than 36 weeks, with newborns having an Apgar Index between 1-8 and weights between 1880 g and 2500 g. Paraclinically, a newborn showed changes in myocardial enzymes on the first day postpartum (myoglobin 170.8 ng/mL, CK-MB 8.5 ng/mL), without clinical symptoms.

Samples were taken to identify SARS-CoV-2 from amniotic fluid, placental blood, oropharynx in newborns and breast milk, with negative results, without proving a vertical path of transmission.

The limitations of this study are represented by the low number of patients, as well as their enrollment in the third trimester, being unable to prove the possibility of vertical transmission in the first or second trimester, when the transmission rate in viral infections is higher.

Another limitation of the study could be considered the birth by cesarean section, taking into consideration the variant of vertical transmission, given the tropism of coronavirus for vaginal mucosa cells.

Only one of the nine newborns tested positive in the first 36 hours postpartum.The positive diagnosis was supported by pharyngeal exudate, but vertical transmission could not be demonstrated because no samples were taken from amniotic fluid or placental blood, which is another limitation of the study.⁷

We can hypothetically discuss the ways of vertical transmission of SARS-CoV-2 infection.One of the possible ways is hematogenous. Once it enters the maternal organism, the virus replicates, creating a viremia, it binds to specific receptors, mainly ACE2 receptors, in target organs. At the same time, the hypothesis of crossing the feto-placental barrier by IgM-type immunoglobulins, which could have pathogenic effects on the fetus, must be taken into account, given that IgM immunoglobulins have a lower molecular weight than IgG, so they could easily cross the feto-placental barrier.

The ascending path of transmission was also considered, by local contamination, either from the vaginal mucosa or through feces, in the case of membranes spontaneously ruptured for a long period of time, in which case the effect could be the appearance of chorioamnionitis.

The pathogenicity of coronavirus is achieved by complex mechanisms, both directly and indirectly.⁴

It is directly produced by binding to ACE2 membrane enzymes, producing vasoconstriction, which will later lead to acute or chronic placental lesions such as ischemia.

Indirect pathogenicity occurs in several situations;for example, in the case of prolonged maternal hypoxia, a significant systemic inflammatory reaction is triggered, producing excess cytokines, which will cross the placenta and cause acute fetal distress.

Another pathogenic mechanism is thrombotic, which can cause placental ischemic damage, which will inevitably lead to utero-placental insufficiency, chronic fetal distress, fetal hypoxemia and intrauterine growth restriction.Cases of SARS-CoV-2 in pregnant women, reported at the onset of the pandemic, describe placental changes such as increased fibrin deposits in the villi, chorionic hemangiomas and a case of massive placental infarction, but no case of chorioamnionitis has been reported, however, these cannot be ruled out in the case of an ascending infection.⁸

A study of 55 pregnant women in the Department of Obstetrics and Gynecology, Faculty of Medicine, National University of Singapore, developed a hypothesis about the protective role of T-helper 2 lymphocytes on the fetus. During pregnancy, physiological and mechanical changes predispose the pregnant woman to infections, mainly affecting the cardiovascular and respiratory system, with evolutionary potential for severe cardio-respiratory failure. It has been noted that Th2 lymphocytes have a more important expression in the fetal immune system, compared to T-helper 1 lymphocytes, which predominate in pregnant women and this imbalance is the reason why they are more susceptible to viral infections, but the fetus remains protected.⁹

Regarding the evidence of vertical transmission of SARS-CoV-2 infection, the time of its occurrence must be analyzed, so that we can talk about congenital infection, peripartum infection and neonatal infection.¹⁰

In the first trimester of pregnancy, infection with the new coronavirus can cause miscarriage or teratogenic effects. The same pattern is observed in most viral infections, for example in rubella, pathogenicity is much higher in the first trimester, teratogenic effects are considerably lower in the third trimester.¹¹

If the PCR is positive for samples taken from both the maternal and fetal surfaces of the placenta, we can speak of a congenital infection that can result in death in utero.¹²

In symptomatic cases, congenital infection is confirmed by detection of the virus by PCR from placental or fetal blood, collected in the first 12 hours postpartum or from amniotic fluid, taken before rupture of the membranes.

Neonatal infection can be acquired intrapartum. In the case of symptomatic newborns from infected mothers, intrapartum infection is confirmed by RT-PCR positive for SARS-CoV-2 harvested from the nasopharynx in the first 24 to 48 hours postpartum. However, neonatal infection can also occur in the immediate postpartum period. Thus, it is defined by clinical criteria specific to COVID-19 infection started at over 48 hours of age, without taking into consideration the viral status of the mother and negative PCR test at birth, but with its positivity, with samples from nasopharyngeal exudate or rectal.¹³

According to the study conducted at Wuhan University Hospital, of the nine cases of pregnant women testing positive for SARS-CoV-2 infection in China, Zhongnan region, at the beginning of the pandemic, none resulted in maternal-fetal death.

Although vertical transmission of the new coronavirus infection could not be proven, there was an increased risk of premature rupture of membranes, premature birth, fetal tachycardia, and acute fetal distress if the infection occurs in the third trimester. This supports the hypothesis of a potential teratogenicity of the virus if the infection occurs in the first trimester of pregnancy.^7,14

Given the relatively small number of SARS-CoV-2 infections in pregnancy and the limited information about the maternal-fetal transmission of the new coronavirus, we decided to make a parallel, highlighting the pathogenic impact on pregnant women and fetuses of SARS-CoV-1 infections, MERS and SARS-CoV-2.

We identified a significant number of relevant case studies on which a meta-analysis was performed, including 19 studies targeting 79 women, counting 41 pregnant women affected by COVID 19 (51.9%), 12 infected with MERS (15.2%), and 26 with SARS (32.9%).¹⁵

The perinatal effects observed were acute fetal distress, Apgar index less than 7 at 5 minutes, neonatal asphyxia, need for care in neonatal Intensive Care units, perinatal death and evidence of vertical transmission.

Comparing with the pathogenic effect of SARS and MERS infections, similar developments were described in SARS-CoV and MERS-CoV-positive pregnant women, being reported first-trimester miscarriage (50%), intrauterine growth restriction (40%), premature birth (80%), perinatal death (Figure 1).^14,15

For SARS-Cov-2, all vaginal births were carried out in accordance with internationally established limits in order to prevent the transmission of the infection: mothers wore the N95 mask and were isolated from newborns immediately after birth.¹⁶

In conclusion, a certainty diagnosis is difficult to establish, due to the low number of infections in pregnant women, but also by the limitations of test methods: once by the relatively low sensitivity and specificity of tests (RT-PCR - sensitivity 70%, immunological: antibody determination IgM-type antibodies have a specificity of 80-95%, and in the case of IgG-type antibodies, the sensitivity increases over time and inversely proportional to the symptoms). Another limitation in establishing a positive diagnosis is the difficulty of visualizing the virus in electronic microscopy, which is why identifying susceptible fetal organs, lung tissue, heart, digestive tract, through immunohistochemistry studies, could be a solution to highlight coronavirus infection and a step forward in demonstrating the hypothesis of vertical transmission.

To support maternal-fetal transmission, it is very important to specify that samples should be taken from amniotic fluid, placental blood, vaginal mucosa, in order to identify viral RNA in the immediate postpartum, fetal nasopharyngeal exudate being insufficient.

Another method of diagnosis of vertical transmission could be represented by the systematic histopathological examination of the placenta, which was however not practiced in all cases.

An aspect worth to be researched in the future would be the degree of impairment of the newborn in conditions of severe maternal hypoxemia, as well as maternal morbidity and mortality, but also neonatal.

The evolution of SARS-CoV-2 infection in pregnant women also depends on the associated comorbidities, so that in the case of the association of pathologies such as gestational diabetes, pre-existing or pregnancy-induced hypertension, kidney damage, obesity, when, however, immune status is compromised, there is a susceptibility to severe forms, which can result in maternal death.

In the absence of clear evidence of vertical transmission, it is necessary to monitor the pregnant woman with COVID-19, for ante- and postpartum infection.

It is not possible to discuss an optimal way of giving birth, as there is no benefit for Cesarean section, if the pregnant woman is asymptomatic and there is no degree of fetal distress, because studies performed in vaginal births did not indicate a clear transmission of peripartum infection. However, isolation of the newborn is preferred and breastfeeding is contraindicated.

Despite the growing number of studies in pregnant women with SARS-CoV-2 infection, conclusive data to support the hypothesis of vertical transmission are insufficient, as well as those targeting the severity and specific complications of the infection in both pregnant females and newborns.

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