Health chiefs in the north-east of England said they were concerned by the level of take-up of flu jabs by pregnant women.
NHS North East said figures showed about 20% of the estimated 20,000 pregnant women had received the jab.
It warned flu could cause serious illness in pregnant women and in severe cases put the lives of mothers and their unborn babies in serious danger.
It said the vaccine could be given at any stage of pregnancy.
NHS North East deputy medical director Dr Tricia Cresswell said: "Pregnant women will do anything to protect their unborn child and I really would urge them to get vaccinated as a matter of priority by speaking to their midwife or GP.
"The flu vaccine is safe for mother and baby and can be given at any stage of pregnancy.
"The earlier you have the vaccine the better as it means you will be protected for the whole winter and it may also give your unborn baby some protection against flu during the first six months of life."
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regulatory consultants