What will happen if I don't receive a tetanus booster during my pregnancy?

Depends. I guess you are really referring to the use of tdap during the 2nd or 3rd trimester to boost immunity to pertussis. Pertussis (whooping cough) has been on the rise in the usa and elsewhere and infants are a major target with a real risk of severe disease or death. Immunizing the pregnant woman will help 'cocoon" the baby against the disease.