Group B Strep (short for streptococcus) is caused by a bacteria that lives in people’s gastrointestinal and genital tracts and around 1 in 4 pregnant women carry the bacteria. This bacteria can spread—but how people spread remains unknown. Most people who carry the bacteria have no symptoms, so they don’t even know they have it—which is why your ob-gyn will test you for it, and it’s typically not dangerous (in rare cases it can cause infections). However, it’s known that women with Group B Strep can pass it to their babies during delivery.