Why Not To Get Tdap While Pregnant? The Science Behind It

The Tdap vaccine is recommended during every pregnancy, usually between 27 and 36 weeks, and that recommendation is one of the most consistently supported in prenatal care. The reason is straightforward: newborns cannot receive the Tdap vaccine themselves, and whooping cough can be deadly in the first months of life. Vaccinating the mother near the end of pregnancy causes her body to make antibodies that cross the placenta, giving the baby some protection until they are old enough to be vaccinated directly. So the question “why not to get Tdap while pregnant” has a short answer — for most pregnant women, the established clinical guidance is to get it, and the reasons people give for avoiding it do not hold up against the evidence.

That does not mean every concern is unreasonable, and it does not mean the vaccine is a perfect shield. It means the tradeoffs have been studied in large populations, and the balance of benefit and risk points toward vaccination. This article explains what the vaccine does, what the actual evidence shows about safety and effectiveness, and where the honest limits of that evidence lie.

What Does the Tdap Vaccine Actually Protect Against?

Tdap is a combination vaccine that protects against three bacterial diseases: tetanus, diphtheria, and pertussis. Pertussis is the one that matters most in pregnancy. It is also called whooping cough.

Pertussis is a respiratory infection caused by the bacterium Bordetella pertussis. In adults and older children it often looks like a bad cough that drags on for weeks. In newborns it can look very different — sometimes just pauses in breathing, called apnea, rather than a classic cough. Infants under 2 months old are at the highest risk of severe illness, hospitalization, and death from pertussis, and they are too young to be vaccinated themselves. The first routine pertussis vaccine dose is not given until around 2 months of age, and protection is not complete until a series of doses is finished.

That gap — roughly the first two months of life — is the entire reason maternal vaccination exists. The mother’s antibodies are the only vaccine-derived protection a newborn can have during that window. This is sometimes called “cocooning” when it refers to vaccinating everyone around the baby, but maternal vaccination is a different and more direct strategy: it passes protection through the placenta before birth rather than relying on others to avoid spreading infection.

Why Not To Get Tdap While Pregnant — Where Does That Concern Come From?

Most hesitation traces back to a few recurring claims. It is worth looking at each honestly.

The vaccine gives the baby pertussis. Tdap contains inactivated or purified components of the bacteria. It cannot cause pertussis, because it does not contain live bacteria. This is not a matter of debate — it is a property of how the vaccine is made.

Getting it during pregnancy is new and untested. The recommendation to give Tdap in every pregnancy has been in place in the United States since 2012, and the vaccine itself has been used for far longer. That is more than a decade of use across millions of pregnancies, with safety monitoring systems specifically designed to detect problems after a vaccine is licensed.

It is too many vaccines at once. Tdap is a single injection. It does not add a large number of separate shots to the prenatal schedule.

The mother already had a Tdap shot. Immunity from pertussis vaccination fades over time. A dose given years ago, or even in a previous pregnancy, does not reliably protect a new baby. That is why the recommendation is repeated each pregnancy rather than given once.

These claims circulate widely, but none of them is supported by the clinical evidence. The genuine scientific questions are narrower and more interesting than the viral ones.

What Does the Evidence Show About Safety During Pregnancy?

The safety data come from several directions: pre-licensure studies, large observational studies of pregnant women who received Tdap, and passive reporting systems that track adverse events after vaccination.

Across these sources, the most commonly reported side effects are the same ones seen in non-pregnant people — soreness at the injection site, fatigue, headache, and mild fever. These are short-lived. Serious adverse events are rare, and studies have not identified a consistent link between Tdap in pregnancy and problems such as preterm birth, low birth weight, or birth defects.

One point deserves care here. “No link found” is not the same as “proven perfectly safe in every possible way.” Large observational studies can miss very rare events, and they cannot rule out small effects with certainty. What can be said honestly is that the evidence base is substantial, the findings are consistent, and no safety signal has emerged that outweighs the benefit. That is a strong position — stronger than most medical interventions in pregnancy can claim.

Some clinicians and researchers have raised a separate question about timing and antibody interference: could maternal antibodies blunt the baby’s own response to their later vaccine doses? This has been studied, and the pattern seen is a modest reduction in some antibody levels after the infant series, not a loss of protection. The clinical significance of this is still being followed, and it has not changed the recommendation. It is a real scientific question, unlike the claims above.

How Well Does It Actually Protect a Newborn?

This is where the evidence is strongest and also where it is most often oversold.

Maternal Tdap vaccination during pregnancy reduces the risk of pertussis in infants too young to be vaccinated. Multiple studies, including large ones conducted during pertussis outbreaks, have found a meaningful reduction in infant cases. The protection is not complete, and it is not permanent. It is highest in the first weeks of life and wanes as the baby’s own immune system develops and the maternal antibodies fade.

Timing matters. The recommendation is to give Tdap between 27 and 36 weeks of gestation. This window is chosen because it allows enough time for the mother to build antibodies and for those antibodies to cross the placenta, while still being close enough to birth that antibody levels are high when the baby arrives. Giving it earlier in pregnancy may allow antibody levels to decline before delivery. Giving it after 36 weeks may not leave enough time for the full response.

This is why the vaccine is not a substitute for the infant’s own vaccine series. It is a bridge. The baby still needs their routine pertussis vaccinations on schedule starting at around 2 months.

Is There Any Situation Where Tdap in Pregnancy Should Be Delayed or Avoided?

There are genuine contraindications, but they are uncommon.

A severe allergic reaction — anaphylaxis — to a previous dose of Tdap or to any component of the vaccine is a reason not to receive it. A severe allergy to any tetanus, diphtheria, or pertussis-containing vaccine is the same. These are the standard contraindications listed for the vaccine in general, not pregnancy-specific.

Moderate or severe illness with fever at the time of the appointment is usually a reason to wait until the person has recovered, which is standard practice for most vaccines.

For anything beyond these, the answer depends on the individual. A pregnant woman with a history of severe allergic reactions, a compromised immune system, or another complex condition should discuss timing and suitability with her own clinician rather than relying on general guidance. That is not a hedge — it is how individualized medical decisions work.

The Bottom Line on the Science

The case for Tdap in pregnancy rests on a clear chain of established facts: newborns are vulnerable to pertussis, they cannot be vaccinated in the first months of life, maternal antibodies cross the placenta, and vaccination during a specific window raises those antibody levels in time for birth. Large studies support both the safety profile and the protective benefit.

The reasons people give for avoiding it — that it causes the disease, that it is untested, that it is unnecessary if you were vaccinated before — are not supported by the evidence. The honest scientific questions that remain, such as how maternal antibodies interact with the infant’s later vaccine response, are being studied and have not changed the recommendation.

None of this is a reason to skip a conversation with your own clinician. It is a reason to have that conversation with accurate information rather than viral claims.