Pregnant women in South Australia are being offered the whooping cough vaccination for free by the State Government in a bid to prevent an outbreak of the deadly illness.

A four-week-old baby died in Western Australia last week from the infectious disease and there are concerns about a spike in the number of cases across the country.

Health Minister Jack Snelling said he hoped as many expectant mothers as possible would take advantage of the free vaccine.

"As we saw last week, whooping cough is a very serious illness that can have tragic and devastating consequences for those who have not been fully immunised," Mr Snelling said.

"Infants under six months of age are not able to complete the required series of vaccinations so they remain especially vulnerable to whooping cough.

"By giving the mother a whooping cough vaccination in the last trimester of pregnancy, this immunity is transferred to the baby, providing over 90 per cent protection."

SA Health chief medical officer Paddy Phillips said while the number of cases in South Australia were relatively low, the cyclical nature of whooping cough meant an increase in cases was likely.

"Whooping cough is a serious respiratory infection, which is particularly dangerous in very young infants," Professor Phillips said.

"We know the number of whooping cough cases increases sharply every four to five years so it's important that we prepare ourselves now for a spike in numbers."

Megan O'Dowd is 28 weeks' pregnant, and has opted to take advantage of the free vaccination.

"There's been a lot more cases of it recently, and with the recent case of Riley passing away, it's very scary," Ms O'Dowd said.

"Personally, I think all kids should be vaccinated.

"We can't rely on this herd immunity anymore because a lot more people aren't getting immunised, so herd immunity is not working."

Ms O'Dowd encouraged other expectant mothers to make use of the free vaccinations.

"I'd even encourage fathers, grandparents, anyone who's going to be spending time around your baby to have them," she said.

"It's not very difficult, it's pain-free so why wouldn't you do it?"