Why do doctors only test for step b in the 3rd trimester and not in the second trimester?

Risk in delivery: Many (about 25%) of women just carry group b strep naturally, not as an infection. However, this colonization can cause an infection in the baby during delivery, so you check shortly before and decolonize. Decolonizing early just gives it more time to re-establish colonization again.

Timing: Group b strep is intermittently present in the genital tract. It is not a problem for the woman, but can cause serious infections in babies that are infected during birth. Early testing does not adequately predict presence or absence of the bacteria at the time of birth. The appropriate timing is 35-37 weeks unless there is already a reason for preventative antibiotics during labor.