Pregnant women should be sure to get the tetanus-diphtheria-pertussis (Tdap) vaccine during pregnancy, since it is highly effective in protecting newborns against pertussis (whooping cough) in the first two months of life, a new study found.

Infants can receive a version of the vaccine when they are 2 months old, but until then they are unprotected against whooping cough.

Researchers followed 148,981 babies born to mothers in a large health maintenance organization in California between 2010 and 2015 until they were 1 year old. About a quarter of the mothers were unvaccinated when their babies were born.

Of the 103 cases of whooping cough in infants that were reported after one year, 80 were in babies of mothers unvaccinated during pregnancy. The observational study is in Pediatrics.

The Centers for Disease Control and Prevention recommends that all pregnant women receive the Tdap vaccine; it is more than 90 percent effective in protecting infants in the first two months of life. Nationwide in 2015, less than half of pregnant women had received it.

The senior author of the study, Dr. Nicola P. Klein, a research scientist at Kaiser Permanente of Northern California, said that “every woman should receive the Tdap during every pregnancy. Whooping cough is a potentially deadly disease, and this vaccine protects your baby.”