Group B strep and the antibiotics in labor
A genuine benefit, and an honest limit.
Group B streptococcus is a bacterium carried harmlessly by roughly a quarter of women. Passed to a baby during birth it can occasionally cause serious infection. A vaginal and rectal swab is taken at around 36 weeks, and women who carry it are given antibiotics through a drip during labor.
Early-onset group B strep disease fell from about 1.7 cases per 1,000 live births in the early 1990s to about 0.2 per 1,000 recently, roughly an 80% to 90% reduction, over the period in which screening and antibiotics became standard. That is a before-and-after comparison across a population rather than a randomized trial, so it is strong evidence but not proof that the policy alone produced the fall.
The honest limit: in a surveillance study of 217,480 newborns, more than half of the group B strep cases that did occur, 24 of 45, were in babies whose mothers had screened negative. Screening and antibiotics reduce the problem substantially. They do not eliminate it.