Box 1. Conclusions from systematic reviews, narrative reviews and large observational studies identified in the present review.

Systematic reviews
Di Mascio et al. (2020): 17 “In mothers infected with coronavirus infections, including COVID-19, > 90% of whom also had pneumonia, PTB is the most common adverse pregnancy outcome. Miscarriage, preeclampsia, cesarean, and perinatal death (7-11%) were also more common than in the general population. There have been no published cases of clinical evidence of vertical transmission.”
Della Gatta et al. (2020): 18 “The available data on COVID-19 illness in pregnant patients do not provide a clear conclusion into the clinical implications for mother and fetus. The outcome thus far described is favorable, but fetal and maternal risks should be underestimated. Although preterm delivery was mostly the consequence of elective interventions, a trend towards spontaneous prematurity is present. It is essential that future studies provide more detailed information on maternal and fetal conditions, as well as the rationale for obstetric interventions. Experience, thus far, is limited to patients that developed the disease in late gestation and were delivered shortly after the diagnosis. The fetal consequences of long-standing infections occurring in early gestation are unknown.”
Zaigham and Andersson (2020): 19 “Current evidence suggests the possibility of severe maternal morbidity requiring ICU admission and perinatal death with COVID-19 infection in pregnancy. Maternal-fetal transmission of the SARSCoV-2 virus was not detected in the majority of the reported cases, although one neonate had a positive qRT-PCR 36 hours after birth despite being isolated from the mother. Careful monitoring of pregnancies with COVID-19 and measures to prevent neonatal infection are warranted.”
Abdollahpour and Khadivzadeh (2020): 20 “No trustworthy evidence is available yet to support the possibility of vertical transmission of COVID-19 infection from the mother–baby. Mother-to-child transmission of respiratory viruses mostly happens via the birth canal and during breastfeeding or close contact among health care providers, family members. To our knowledge, no article has reconnoitered that reporting a newly vertically transmitted case.”
Banaei et al. (2020): 21 “There was some evidence about neonates COVID-19 in the included studies, but it is not clear whether the source of the infection in these neonates is from the mother or from the environment. In the majority of studies, there was no evidence of vertical transmission. In most studies, the neonates were separated from the mother after birth to reduce the chance of transmission, but there is also currently insufficient evidence regarding the mother/baby separation. If the mother is severely or critically ill, separation should be considered. The result of a review showed that in SARS vertical transmissions were not seen.
Duran et al. (2020): 22 “There is still no evidence supporting vertical transmission of COVID-19. Some newborns were positive for COVID-19 in spite of the reported use of preventive measures during and after delivery, but even in these cases there was no evidence supporting vertical transmission.”
Elshafeey et al. (2020): 23 “We extracted data regarding potential vertical transmission. In four neonates who had RT-PCR confirmed infection, samples from cord blood and amniotic fluid were negative. Based on the available data, we are uncertain of the mode of transmission, since there is no evidence that these four cases were the result of a vertical transmission.”
Gordon et al. (2020): 24 “Neonatal infection is uncommon, with only two previously reported cases likely to be of vertical transmission. The case we report is still RT-PCR-positive on day 28, and is asymptomatic.”
Huntley et al. (2020): 25 “Data from early in the pandemic is reassuring that there are low rates of maternal and neonatal mortality and vertical transmission with SARS-CoV-2.”
Juan et al. (2020): 26 “Despite the increasing number of published studies on COVID-19 in pregnancy, there are insufficient good-quality data to draw unbiased conclusions with regard to the severity of the disease or specific complications of COVID-19 in pregnant women, as well as vertical transmission, perinatal and neonatal complications.”
Kasraeian et al. (2020): 27 “Currently, no evidence of vertical transmission has been suggested at least in late pregnancy. No hazards have been detected for fetuses or neonates. Although pregnant women are at an immunosuppressive state due to the physiological changes during pregnancy, most patients suffered from mild or moderate COVID-19 pneumonia with no pregnancy loss, proposing a similar pattern of the clinical characteristics of COVID-19 pneumonia to that of other adult populations.”
Ludvigsson (2020): 28 “Newborn infants have developed symptomatic COVID-19, but evidence of vertical intrauterine transmission was scarce.”
Muhidin et al. (2020): 29 “No fetal infection through intrauterine vertical transmission was reported.”
Smith et al. (2020) 30 : “It is unclear if this is evidence of vertical transmission or if it was contracted post-delivery due to delayed RT-PCR testing 36 hours from birth. The evidence for vertical transmission appears equivocal.”
Walker et al. (2020): 31 “To date, there have been 28 cases published where the possibility for vertical transmission to have occurred have been reported. To confirm definite vertical transmission, it has been proposed that detection of the virus by PCR in either umbilical cord blood, neonatal blood collected within the first 12 hours of birth, or amniotic fluid collected prior to rupture of membranes is needed. In no cases reported to date have these criteria been met although some report negative testing. A few cases deserve special mention: one case reports a positive nasopharyngeal swab in the neonate on the day of birth. The authors do not describe any procedure or care taken to clean the infant's oropharynx / mouth/nares / face prior to procuring the swab and we speculate that the presence of the virus may be due to contamination by maternal stool. Of note, the virus was not detected on repeat swab and the infant remained well. The presence of IgG would be maternal, so again not diagnostic. The UKOSS study reports 12/24 cases of possible vertical transmission. Limited information is given for the 12 neonates but 6/12 infants tested positive for COVID-19 within 12 hours of birth. It is unclear what method of testing was used and if this was a nasopharyngeal swab without precautions to clean the infant prior to testing, may again be a result of contamination. In another case, a positive nasopharyngeal swab in the neonate on the day of birth occurred after careful separation of the baby and cleansing of the baby prior to taking the swab.”
Yang and Liu (2020) 32 : “There is currently no direct evidence to support intrauterine vertical transmission of SARS-CoV-2. Additional RT-PCR tests on amniotic fluid, placenta, and cord blood are needed to ascertain the possibility of intrauterine vertical transmission. For pregnant women infected during their first and second trimesters, further studies focusing on long-term outcomes are needed.”
Yang et al. (2020): 33 “Currently, there is no direct evidence suggesting that COVID-19 in pregnancy could lead to fetal infection via intrauterine vertical transmission. Long-term outcomes and potential intrauterine vertical transmission need further analysis.”

Abbreviations: ICU, intensive care unit; IgG, immunoglobulin G; PTB, preterm birth; qRT-PCR, real-time quantitative polymerase chain reaction; RT-PCR, real-time polymerase chain reaction; SARS, severe acute respiratory syndrome ; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; UKOSS, United Kingdom Obstetric Surveillance System.