For most vaccines targeted to pregnant women, vaccination would be given early in the third trimester (28–32 weeks of gestation), a time when organogenesis is complete and when most events associated with adverse pregnancy outcomes have passed. This timing would also theoretically provide protection for many prematurely born infants. Early in the third trimester is also a period when most pregnant women encounter the health care system. Preconceptional or adolescent encounters with the health care system are far less frequent, and efforts targeted at nonpregnancy-related health care visits would result in much lower rates of immunization than efforts targeted at pregnant women in the third trimester.

Because of the exaggerated concerns about the use of vaccines during pregnancy, the most frequent suggestion is to provide immunizations prior to childbearing. This approach requires special access to the health care system, whereas immunization during the third trimester would use existing access mechanisms and assumes that the antibody response would persist for several years at levels ensuring protection for the infant. Many experts have suggested vaccine administration around the age of puberty, and with the recommendation for booster doses of vaccines, including those for measles, mumps, rubella, and tetanus. At this age, an existing medical access system could be used. However, if a girl was immunized at age 12 years with a quadravalent group B streptococcal glycoconjugate vaccine, for example, and had her first pregnancy at age 32 years, several problems would be expected. First, it is unlikely that 20 years later levels of antibodies to this pathogen would be sufficiently high to ensure protection of the infant. Second, a small number of girls would become pregnant before the “adolescent” or “puberty” immunization visits. Third, if proof of vaccine efficacy were to require testing of adolescents and observation through the first pregnancy, the logistics and expense of such a study would likely be an even greater deterrent to the pharmaceutical industry than considering a trial that would immunize women in the third trimester.