WASHINGTON - Norwegian pregnant women who received a vaccine against the 2009 H1N1 influenza virus showed no increased risk of pregnancy loss, while pregnant women who experienced influenza during pregnancy had an increased risk of miscarriages and still births, according to a study to be published online Thursday in the New England Journal of Medicine.h The research was conducted by scientists at the US National Institutes of Health and the Norwegian Institute of Public Health (NIPH), following the H1N1 influenza pandemic that took place between spring 2009 and fall 2010.

Norwegian public health officials had urged pregnant women to be vaccinated. However, media reports of pregnancy losses after flu shots caused some expectant mothers to forgo vaccination. First author Siri Haberg, of the NIPH and colleagues initiated the study to help address the question of vaccine safety, by taking advantage of Norway's excellent registries and medical records system.

The researchers combined data from obstetrical visits, birth records, and vaccination registries to investigate whether the influenza vaccination posed a risk to pregnancy. The study found that influenza infection increased the risk of fetal loss by up to twofold. Influenza vaccination did not increase the risk. Instead, the results suggest that vaccination reduces the risk of fetal loss.

"Pregnant women should find it reassuring that we found no harmful effects on the fetus associated with H1N1 vaccination," Haberg said.