A Tdap injection is recommended anytime from 16 weeks to delivery. It contains a vaccine against two viruses - tetanus and pertussis (whooping cough). Babies are not vaccinated till they are several months old, so for the first few months are not protected. When the mother gets vaccinated, she will produce her own antibodies against these viruses, and these antibodies are passed on to the baby, providing protection for the first few months before getting vaccinated.