Tdap protects against tetanus, diphtheria and pertussis (whooping cough).
Pertussis can be particularly serious for young babies, especially before they are old enough to receive their own vaccines. Vaccination during pregnancy allows antibodies to pass from mother to baby through the placenta, helping provide early protection.
FOGSI guidance places Tdap in the third trimester, with commonly recommended timing around 27–36 weeks.
If you're unsure whether you've already received it, don't guess - ask your obstetrician.