They don’t do clinical trials on pregnant women which is why the insert says that. But they do plenty of retroactive studies on pregnant women and there is no risk to you or baby. TDap is important to have before baby is born so you pass immunity down to the baby before he/she is born to prevent them from getting a deadly and horrible disease. Whooping cough is unfortunately very prominent now specifically because of anti-vaxxers. Same with measles, mumps, polio (not in US) etc. these diseases are preventable and it works best with herd immunity so those who can’t get the vaccine due to ALLERGIES or other REAL MEDICAL ISSUES (not just because you read an insert and don’t feel comfortable) are also still protected because the diseases become less prevalent in society