New and revised recommendations regarding Whooping Cough immunisation in pregnancy have advised that a booster be given in every pregnancy (even if you have had one relatively recently). The timing is ideally near the start of the third trimester (eg 28 to 32 weeks gestation).
The rationale for this new recommendation is that your baby is very vulnerable to whooping during the first couple of months of life (until the first immunisation is given: 6 weeks). If the mother has high levels of antibodies then these will cross through the placenta to the baby and give the baby protection through the first few weeks of life.
Whooping cough booster is safe in pregnancy and whooping cough is very dangerous for newborns, so make sure to get yours done.