Group B Streptococcus (GBS) is a common bacterium that lives naturally in the gastrointestinal and genitourinary tracts of many healthy adults. While usually harmless to the carrier, GBS can pose a risk to newborns during labor and delivery. The prenatal GBS test identifies colonized pregnant individuals so that preventative measures can be taken. This status identification allows healthcare providers to implement a strategy that significantly reduces the chance of the bacteria passing to the infant and causing a serious infection.
Understanding the GBS Screening Process
The GBS screening is a routine part of prenatal care, typically performed late in the third trimester of pregnancy. The testing window is generally between 36 weeks and 0 days and 37 weeks and 6 days of gestation. This timing is chosen because GBS colonization can be transient, meaning the bacteria can come and go, and a result from this specific window is considered the most predictive of the status at the time of delivery.
The collection process is simple and non-invasive, involving a single swab of two areas. Healthcare providers obtain a specimen by swabbing the lower vagina and the anorectal area. This combined swab is then sent to a laboratory for culture, which maximizes the chances of detecting the bacteria if present.
Decoding Your Test Results
Your lab report will communicate one of three main outcomes regarding your GBS status. The most definitive results are “Negative” or “Positive,” which guide the management plan during labor. The GBS test determines if you are a carrier of the Streptococcus agalactiae bacterium, not if you have an infection or illness.
A “Negative” result indicates that the GBS bacteria was not detected in the vaginal and rectal swabs at the time of testing. This result suggests a very low risk of passing the bacteria to the newborn, and no specific action is typically required during labor. However, a negative result is only valid for five weeks, and a small possibility remains that the bacteria could colonize later.
A “Positive” result means the GBS bacteria were successfully grown from the sample, confirming colonization. This is not considered an infection requiring immediate treatment; rather, it signifies an increased risk of transmitting the bacteria during birth. Receiving a positive result triggers specific preventative care during labor, which is highly effective in protecting the baby.
In rare instances, the result may be reported as “Inconclusive” or “Unreadable” due to issues such as poor sample quality or technical problems in the laboratory. In this situation, healthcare providers will usually decide on a treatment plan based on other risk factors, or they may attempt to repeat the test if there is enough time before delivery. The management approach follows current guidelines established by public health organizations like the Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG).
Managing a Positive GBS Status During Labor
A positive GBS test result leads directly to a preventative treatment plan that begins once labor starts or the water breaks. This strategy involves the administration of intravenous (IV) antibiotics to the pregnant patient during the birthing process. The goal of this treatment is to reduce the concentration of GBS bacteria in the birth canal, thereby minimizing the infant’s exposure.
The standard antibiotic used for this purpose is Penicillin G, although Ampicillin is an acceptable alternative, both given through an IV line. For the treatment to be considered fully effective, the antibiotics must be administered for at least four hours prior to delivery. This duration ensures that the medication reaches adequate levels in the blood and surrounding tissues.
Antibiotic treatment is also recommended in certain situations even if the most recent GBS swab was negative or was not done. These exceptions include:
- Having GBS detected in the urine at any point during the pregnancy.
- Having previously delivered an infant who developed GBS disease.
- Going into labor prematurely (less than 37 weeks gestation) with unknown GBS status.
- Having a prolonged rupture of membranes for 18 hours or more with unknown GBS status.