High incidence of maternal parvovirus B19 infection in a large unselected population-based pregnancy cohort in Norway

•High incidence of viremic B19V infections and seroconversion rates were recorded in this cohort of randomly selected pregnant women.•A combination of serology and PCR is essential for diagnosing parvovirus B19 infection in pregnant women.•Lack of seroconversion despite longstanding viremia emphasizes the importance of including PCR when testing for infection during pregnancy.•Vertical transmission of B19V was seen in 20% of mothers with confirmed infection and in 28% of B19V viremic mothers. Around 40% of pregnant women in Norway are parvovirus B19 (B19V) seronegative and thus at risk for B19 V infection. Studies on samples from women with symptomatic disease or known exposure have shown that nucleic acid amplification assays combined with serology increase the sensitivity and improves the diagnostic procedure. The aim was to investigate the seroprevalence of B19V infection, the occurrence of new infections and vertical transmission in a population-based pregnancy cohort, with special emphasis on the diagnostic methods. We randomly selected 1350 pregnant women from the Norwegian Mother and Child Cohort Study (MoBa), using an algorithm for the detection of B19V infection, including both serology and PCR. Maternal infection was confirmed in 50 subjects (3.7% of 1349 women), of which 35(70%) were viremic. Of the initially seronegative 33(6.8%) seroconverted. The estimated average annual seroconversion rate was 15.5%, with the highest estimated annual seroconversion rate of 31.6%. The rates of yearly seroconversion followed the pattern found in reports from Norwegian microbiology laboratories. Among all women, 31 (2.3%) had an inconclusive serological profile and 17 (54.8%) had detectable virus. Of the 16 women with virus detectable at gestational week 17–18, seven were still seronegative with absent seroconversion in the second sample taken at birth. All together 10 children were vertically infected. High incidence of viremic B19V infections and high estimated annual seroconversion rates were found. Lack of seroconversion despite longstanding viremia emphasizes the importance of including PCR when testing for B19V infection during pregnancy.