METHOD
During the second wave of the COVID-19 pandemic in Denmark, we conducted a prospective cohort study entitled the “CareMum COVID-19 study” from August 20th, 2020 to March 1st, 2021, at the Department of Obstetrics and Gynaecology, Copenhagen University Hospital – North Zealand with approximately 4000 annual deliveries.
We included pregnant women who tested positive for SARS-CoV-2 in a pharyngeal swab at a test centre or during a routine test when entering the antenatal clinic or labour ward. Women were eligible if they were to deliver within the study period and were able to give written and oral informed consent in English or Danish. Inclusion took place either during pregnancy at the antenatal clinic or when women were admitted for delivery. All participants had vaginal swabs and maternal blood samples done at the time of inclusion. If included during pregnancy, the samples were repeated at delivery where an umbilical cord blood sample was also taken. At the time of delivery, two vaginal swabs were done – the first, during the initial vaginal examination, and the second, during the active phase of delivery or immediately after. Vaginal swabs were frozen at -20°C until analysis (0-30 days, median 5 days). A maternal blood sample was taken just before or immediately after delivery, and an umbilical cord blood sample was performed immediately after birth. Blood samples were frozen at -80°C until analysis.
In some cases, swabs from the neonate and the placenta, as well as a placental histopathological examination and one fetal autopsy, was performed on clinical indications. Placental swabs were done from both the maternal and the fetal side, and neonatal swabs from the axillary fold, naso-, and oropharynx. These samples were not part of the study protocol, but results are described when present.
Pharyngeal swabs as well as vaginal and placental swabs were analysed by RT-PCR as part of the routine diagnostics (see supporting information, appendix S1, for further information).
Total SARS-CoV-2 antibodies in maternal and cord blood samples were analysed using a qualitative (reactive/non-reactive) biochemical assay, The VITROS Immunodiagnostic Product Anti-SARS-CoV-2 Total (CoV2T), developed by Ortho-Clinical Diagnostics. The test result (S/C) = (signal for test sample / signal at cutoff value) ≥ 1.00 is considered reactive (i.e. positive for antibodies) and result < 1.00 is considered non-reactive (i.e. negative for antibodies).
We obtained demographic and clinical data of participants as well as non-participants in the “CareMum COVID-19 study” from the Danish “COVID-19 in pregnancy” database, which contains information based on medical records on all women diagnosed with COVID-19 during pregnancy in Denmark as described elsewhere (12). Non-participants were women who tested positive for SARS-CoV-2 and gave birth within the study period but who for various reasons were not included in this study (e.g. women did not understand and read Danish/English, did not want to participate or were not asked due to busyness at the maternity ward). Case completeness was secured by a retrospective registry linkage to national databases covering information on results from SARS-CoV-2 pharyngeal swabs. Furthermore, participants were asked to complete a questionnaire about COVID-19 symptoms at the time of inclusion. Symptoms of non-participants were not systematically reported.   
Data was analysed using SPSS version 27 (SPSS Inc., Chicago, IL). Categorical variables are presented as number with percentage and continuous variables as mean with standard deviation (SD) or median with interquartile range as appropriate. Analyses of differences between means, medians and proportions were performed using Student’s T-test, Mann-Whitney, or Fischer’s Exact test, respectively. The study was approved by The Regional Committee on Health Research Ethics.