Strengths and Limitations
To our knowledge, this is the first review looking at the associations between COVID-19 and poor placental perfusion. Both cohorts were well matched for demographic and clinical confounding factors. Our analysis allowed for any seasonal variation in primary outcome.
One limitation of this study is that the sample size is small due to the relative rarity of the primary outcomes. Further analysis would include other disorders of poor placental perfusion (pre-eclampsia, in-utero growth restriction, and preterm birth as well as IUD and placental abruption) over the whole pandemic; however the pandemic is currently ongoing and we report preliminary findings based on the first 6 months. We chose our primary outcomes as potentially acute complications of microemboli/MVM. The other conditions of poor placental perfusion may be relatively chronic in their presentation, and thus the study period would need to be extended to capture these broader complications.
Government policy on (and the availability of) testing for SARS-CoV-2 changed during the time of data collection, and testing for asymptomatic patients was not in our hospital protocol at any point during the review dates. Analysis of patients that test positive for COVID 19 is ongoing via the UKOSS study. In any future reviews of poor placental perfusion, it would be useful to know whether the patients identified were indeed positive for the SARS-CoV-2 virus, and this would potentially allow us to infer causality.
Cases were identified using the hospital reporting system DATIX, which has inherent limitations in terms of identification and reporting. Finally, some of our patients had incomplete documentation. In particular, some of the known haematological and biochemical markers of COVID-19 were not routinely assessed in our asymptomatic populations. (D-dimers, troponin, brain natriuretic peptide, creatinine kinase, ferritin). We are therefore unable to use them as potential surrogate markers for SARS-CoV-2 infection.