**3. Physiology of placenta and placental transmission of COVID-19**

Placenta acts as a barrier for transmission of pathogens by various mechanisms. Placental decidua contains large number of NK cells, macrophages and T cells. Decidual macrophages perform antimicrobial functions while CD8+ T cells protect the fetus from viral infections [8]. Additional protection is provided by syncytiotrophoblast and cytotrophoblast mediated by toll-like receptors. Placenta also has antimicrobial peptides which prevent fetal transmission of various pathogens [9].

The first prerequisite for transmission of any virus through placenta is viremia, which, in case of SARS-CoV2, may be present for a relatively short duration. Although earlier studies documented viremia in only 1–15% of patients infected with SARS-CoV2 virus, recent study using WHO RT-qPCR protocol demonstrated viremia in up to 80% patients, although mostly with low levels of viral load [10].

The second element necessary for fetal transmission is placental tropism or the ability of the virus to infect placental cells. In lungs, SARS-CoV2 uses ACE-2 receptor to enter cells and a serine protease named TMPRSS2 to cleave the spike glycoprotein, facilitating fusion. Although ACE-2 and TMPRSS2 have not been shown to be co-expressed in placental cells, they have been demonstrated in trophoblast, blastocyst and hypoblast. It has been suggested that SARS-CoV2 could enter placenta using other proteases like DPP4, CD147 or trypsin [11].

Another route for this virus to enter the placenta could be through infected blood cells like lymphocytes and macrophages but this has not been proven yet for SARS-CoV2. Transcytosis of free virus particles can infect the fetus, as in case of HIV but this method is thought be less important with maternal SARS-CoV2 infections because of low viremia.

Infection may also cross over to fetus through cervico-vaginal secretions which would raise concerns for the mode of delivery of the fetus. The evidence we have till now does not favour this mode of transmission as one study on ten infected women did not find viral RNA in vaginal secretions in any patient [12].

There is convincing evidence that placental transmission of the virus causing COVID-19 is possible, though rarely. Presence of SARS-CoV2 viral RNA in placenta and SARS-CoV2 virions in syncytiotrophoblast has been reported in multiple cases. IgM antibodies directed against SARS-CoV2 virus have been documented in neonates born to mothers with SARS-CoV2 infection, making a strong case for in-utero transmission of this virus [13].

Placental histology in parturient women who contracted SARS-CoV2 infection during the ante-natal period shows vascular malperfusion, chronic villitis, placental infarcts and fibrin deposition [14].

It has been estimated that maternal to fetal transmission occurs in 3.2% of pregnant women infected with SARS-CoV2 virus. Viral positivity rate for placental and cord samples and fetal serology fall in the same range, supporting the transmission rate of 3.2% [13].

#### **4. Effects of SARS-CoV 2 infection on pregnant women**

Pregnancy is an immuno-deficient state and high morbidity has been reported during previous corona virus outbreaks, both MERS and SARS. Earlier studies done in China showed key outcomes in pregnant women infected with SARS-CoV2 virus to be similar to those in non-pregnant adults [15, 16]. These studies were limited by small sample sizes and a retrospective design. A national analysis of all Covid related ICU admissions in Sweden was one of the first studies to report increased morbidity

during pregnancy [17]. Studies later done in France and the USA supported these findings. The largest study addressing this topic was the review of all laboratory confirmed cases of Covid19 from January to June 2020 in the USA. They found increased risk of hospitalisation, ICU admission and mechanical ventilation but not mortality [18]. This data was recently updated through October 2020 to report increased risk of hospitalisation, ICU admission, mechanical ventilation, extracorporeal membranous oxygenation (ECMO) and death [19]. Though risk appears to be increased when compared with non-pregnant adults, absolute risk remains low.

Clinical course of COVID-19 is believed to be mild in majority (86%) of pregnant females, severe in 9% and critical in 5% [20]. This is similar to incidences reported in non-pregnant Covid19 patients.

In data reported from the UK, most women with more severe illness were in third trimester of pregnancy or postpartum [21]. Risk factors associated with hospital admission in pregnant women include Black, Asian or minority ethnicity, pre-existing co-morbidity (e.g. diabetes, hypertension, asthma), obesity/overweight and maternal age more than 35 years. It has been postulated that this risk is due to genetic differences, socio-economic disparity or difference in response to infection. Vitamin D deficiency has been associated with respiratory infections and ARDS, a common complication of Covid19. South Asian population has been documented to be deficient in Vitamin D and this could be a factor in increased morbidity in this population [21].

COVID-19 increases the risk of thromboembolic complications and pregnancy is itself associated with increased risk of thrombo-embolism. Royal College of Obstetricians and Gynaecologists (RCOG) has emphasised the risk of thromboembolism in COVID-19 with pregnancy and recommends appropriate use of thromboprophylaxis.
