Most women who receive the Tetanus-diphtheria-acellular pertussis (Tdap) vaccine do so after delivery, contrary to recommendations from the Advisory Committee on Immunizations Practices, which calls for vaccination during pregnancy.

Data from the Pregnancy Risk Assessment Monitoring System show that among 6,852 survey respondents in 16 states and New York City who had a live birth between September and December 2011, 20.8% did not know their vaccination status. Of the 5,499 who did know their status, more than half reported being vaccinated with Tdap (13.9% before pregnancy, 9.9% during pregnancy, and 30.5% after delivery).

The findings were published May 21 in Morbidity and Mortality Weekly Report (MMWR. 2015;64:522-6).

ACIP guidelines suggest the optimal time for vaccination is at 27-36 weeks’ gestation, according to the report.

Until 2011, it was recommended that women receive the vaccine either before pregnancy or postpartum. In June 2011, the ACIP changed its recommendation to one dose of the vaccine during pregnancy in women who had never received it before. ACIP further expanded this recommendation in 2012 to recommend vaccination during each pregnancy to provide maternal antibodies for each infant.

“Results from this analysis might reflect the early transition from a policy of vaccinating women postpartum to a policy of vaccinating them during pregnancy,” wrote Indu B. Ahluwalia, Ph.D., and coauthors from the CDC’s Division of Reproductive Health at the National Center for Chronic Disease Prevention and Health Promotion.

Physicians can also “assist pregnant women by providing specific information about where to obtain Tdap vaccination, or offering to provide the vaccination, and also to write a prescription in case it is needed,” the authors wrote.