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Pregnant women who got inoculated with the Tdap (tetanus, diphtheria, acellular pertussis) vaccine did not have an increased risk of giving birth prematurely, a new study reported. The researchers added that the Tdap vaccine was also not linked to a greater risk of giving birth to a small-for-gestational-age infant. The vaccine, however, was tied to a slightly higher risk of being diagnosed with chorioamnionitis, which is a condition characterized by inflammation of the membranes that surround the fetus.

For this study, the team headed by Elyse O. Kharbanda, M.D., M.P.H., of the HealthPartners Institute for Education and Research, Minneapolis analyzed data on 123,494 women who delivered a live birth between January 1, 2010 and November 15, 2012. 21 percent of the women, or 26,229, were vaccinated during their pregnancy.

The researchers found that the vaccine was not linked to an increased risk of preterm delivery or an increased risk of small-for-gestational-age births. Overall, 6.3 percent of women who received the Tdap vaccine and 7.8 percent of women who did not get the vaccine had a preterm birth. 8.4 percent of vaccinated women and 8.3 percent of non-vaccinated women had a small-for-gestational-age birth. The Tdap vaccine did not increase risk of hypertensive disorders.

The team noted that vaccinated women had a higher prevalence of chorioamnionities at 6.1 percent. The rate in unexposed women was just 5.5 percent. The researchers added that the increased risk was very small.

"Given limited prior safety data, continued widespread pertussis transmission, and current recommendations to routinely vaccinate during pregnancy, our study provides important information on the safety of Tdap vaccination during pregnancy," the authors concluded according to the press release.

The Advisory Committee on Immunization Practices recommends the Tdap vaccine for all pregnant women, especially during weeks 27 through 36 of gestation.

The study was published in JAMA.