GBS is a normal flora that lives in the vagina's of many women, however there is evidence that the transmission of GBS to babies during birth can make them very sick. It is for this reason women are screened at around the 36, 37 week mark and if they're GBS positive we give IVAB's during labour and will monitor the baby's temp & respirations for 24 hours after birth. There is some controversy re: GBS screening as, like Ivana_baby said, GBS is transient and 'comes and goes' so where you might be positive at 36 weeks, you could be negative at 39 weeks.