Immunising expectant mums protects their child from whooping cough, a new study found.
Antibodies are passed from mother to the child in the womb helping protect them in the first months of life before they too can be vaccinated.
And the best time for a mother to be immunised is between 27 to 30 weeks of pregnancy to give the best protection to their unborn child.
The risk of whopping cough is highest in infants too young to have completed their primary immunisation series by six months.
In wealthy advanced countries, deaths from whooping cough or pertussis occurs predominantly in infants aged three months or younger.
Giving women in the late stages of pregnancy a Tdap jab for tetanus, diphtheria and acellular pertussis allows them to build up enough antiobodies which they can pass on.
The NHS recommend pregnant women should help protect their babies by getting vaccinated ideally from 16 weeks up to 32 weeks pregnant.
It says if the woman misses having the vaccine, they can still have it up until they go into labour but this was not ideal, as the baby is less likely to get protection.
At this stage of pregnancy, having the vaccination may not directly protect the baby, but would help protect the mother from whooping cough and from passing it on to her baby.
But until now the optimum time of vaccination was unknown.
So the study by Baylor College of Medicine in Houston looked at 626 pregnancies.
It compared pertussis antibody concentrations in umbilical cord blood among newborns whose mothers were vaccinated during weeks 27 through 36 of pregnancy
compared to those who were not.
Having the jab during the third trimester of pregnancy was associated with higher concentrations of pertussis antibodies in infants at birth, with immunisation early in the third trimester associated with the highest concentrations of antibodies.
Assistant Professor Dr Catherine Mary Healy said: “This study demonstrated that immunisation with Tdap vaccine during weeks 27 through 36 of pregnancy, as recommended by the CDC to prevent life-threatening pertussis in young infants, compared with no immunisation was associated with significantly higher concentrations of pertussis toxin antibodies in neonates at birth.
“Concentrations were highest when Tdap vaccine was administered during weeks 27 through 30 and declined thereafter.
“These findings may be important for a number of reasons.
“Pertussis toxin is the pertussis antigen most associated with severe infant disease.
“Although no definitive serologic correlate of immunity for pertussis has been established, it is likely that antibody concentrations needed to protect young infants are higher than those required for older children and adults.
Older children and adults, who are likely primed through prior immunisation, natural infection, or both and whose ability to mount humoral and cell-mediated immune
responses is more developed than in young infants, have alternative mechanisms for mounting a robust immunologic response.
“Pertussis toxin antibody concentrations at birth were sufficiently high in infants born to Tdap-immunised mothers that, even allowing for the natural decay of maternal antibodies, most infants would have had substantial antibody levels until initiation of the primary vaccine series, thus reducing their risk of pertussis-related mortality and morbidity.”
The whooping cough vaccine has been used routinely in pregnant women in the UK since October 2012 and a study by the Medicines and Healthcare Products Regulatory Agency (MHRA) of around 20,000 vaccinated women has found no evidence of risks to pregnancy or babies.
To date, around three fifths of eligible pregnant women have received the whooping cough vaccine with no safety concerns being identified in the baby or mother.
The NHS adds it is highly effective in protecting young babies until they can have their first vaccination when they are two months old.
And babies born to women vaccinated at least a week before birth had a 91 per cent reduced risk of becoming ill with whooping cough in their first weeks of life, compared to babies whose mothers had not been vaccinated.
A number of other countries, including the US, Argentina, Belgium, Spain, Australia and New Zealand, currently recommend vaccination against whooping cough in pregnancy.
The study was published in JAMA.
