There IS research showing that pertussis antibodies are transferred from mother to fetus. The antibodies wane by 2 months of age, but they do provide protection during early infancy. Early infancy is the time that pertussis infections are most serious. Here are links to a few published studies showing this...
Prevalence of Pertussis Antibodies in Maternal Delivery, Cord, and Infant Serum
"These data support the rationale for maternal or neonatal immunization, with acellular pertussis vaccines, to prevent life-threatening pertussis in early infancy."
The case for maternal vaccination against pertussis
"In one study, protection of newborn babies was found. Additional support for the efficacy of maternal vaccination comes from studies showing that transfer of antibodies confers protection against pertussis."
Prevention of Pertussis, Tetanus, and Diptheria Among Pregnant and Postpartum Women and Their Infants
"The results of these surveys suggested that high concentration of transplacental pertussis antibodies might provide a degree of infant protection against pertussis in the first 6 months of life"