Group B strep is a common bacterium that lives in the digestive tract and vagina of many healthy adults. The “NAAT” test is a molecular test that detects the DNA of this bacterium, and it is one of the ways laboratories identify group B strep during pregnancy. Understanding what the test looks for, when it is used, and what a positive result actually means helps you have a clearer conversation with your doctor.
What Is Group B Strep and Why Does It Matter in Pregnancy?
Group B Streptococcus (GBS) is a type of bacteria, not a disease. It is different from group A strep, which causes strep throat. GBS commonly lives in the gut and the vagina without causing any symptoms at all. Carrying it is not a sign of poor hygiene or illness.
Somewhere between roughly one in ten and one in three pregnant women carry GBS at any given time, depending on the population studied and the method used to test. This is a well-documented range in public health literature. Carrying the bacteria is usually harmless to the mother.
The concern is the newborn. During labor, a baby can come into contact with GBS as they pass through the birth canal. In a small number of cases, the bacteria can cause serious infections in the newborn, including pneumonia, meningitis, and bloodstream infection. This is called early-onset GBS disease, and it appears within the first week of life.
Because of this risk, screening pregnant women for GBS is standard practice in the United States. The goal is simple: identify who carries the bacteria so that antibiotics can be offered during labor to reduce the chance of passing it to the baby.
What Is the Group B Strep NAAT Test?
NAAT stands for nucleic acid amplification test. It detects the genetic material, or DNA, of group B strep directly from a sample. If GBS DNA is present, the test can identify it, often faster than traditional methods.
The sample is collected the same way as a standard GBS culture. A swab is taken from the lower vagina and the rectum. This is not a cervical swab and not a urine test, though a urine test may be done separately if a urinary tract infection is suspected.
The difference is what happens to the sample afterward. A culture grows the bacteria in a lab dish over one to two days and then identifies what grew. A NAAT skips the growing step and looks for the bacteria’s DNA instead. This can produce a result in hours rather than days.
Both approaches are used in clinical practice. The choice often depends on what a given laboratory is set up to do and how quickly a result is needed.
How Is the NAAT Test Different From a Standard Culture?
The traditional culture has been the standard method for GBS screening for decades. It is well studied, and it allows the lab to test which antibiotics will work against the bacteria, which is useful for people with a penicillin allergy.
A NAAT does not tell you which antibiotics will work. It only tells you whether the bacteria’s DNA is present. That is a real limitation. If a woman is allergic to penicillin, the culture’s antibiotic-sensitivity information can matter.
On the other hand, NAAT results come back faster. That can be helpful when a woman arrives at the hospital in labor and her GBS status is unknown. In that situation, a rapid test can guide whether antibiotics should be started while waiting for a result.
Neither test is perfect. A NAAT can sometimes detect DNA from bacteria that are no longer alive, and a culture can miss low levels of the bacteria. Both have false positives and false negatives, though rates are generally low for both methods.
When Is the GBS Test Done During Pregnancy?
Standard screening is done late in pregnancy, typically between 36 and 37 weeks and 6 days of gestation. This timing is intentional. GBS carriage can come and go, so a result from early pregnancy may not reflect whether a woman is carrying the bacteria at the time of delivery.
A swab collected close to the due date gives the most useful information. If the test is done too early, and the result is negative, it may no longer be accurate by the time labor begins.
The test is generally offered to all pregnant women, whether they are planning a vaginal delivery or a cesarean. A planned cesarean delivery performed before labor begins and before membranes rupture carries a much lower risk of passing GBS to the baby, but screening is still commonly done because circumstances can change.
What Does a Positive GBS Test Mean?
A positive result means the bacteria were detected in the sample. It does not mean you are sick, and it does not mean your baby will get sick. Most babies born to GBS-positive mothers are healthy.
What it does mean is that antibiotics will likely be offered during labor. The most common approach is intravenous penicillin, given once labor starts and repeated at intervals until delivery. For women with a penicillin allergy, other antibiotics may be used depending on the specific allergy.
Antibiotics given during labor reduce the risk of early-onset GBS disease in newborns. This is one of the more well-established findings in obstetric care. The benefit is greatest when antibiotics are started at least four hours before delivery, which is why timing matters.
A positive result does not usually lead to antibiotics during pregnancy itself, unless GBS is found in the urine. GBS in the urine, called a urinary tract infection, is treated when it is found because it can cause kidney infection and other complications.
Can You Decline the GBS Test?
Screening is a routine recommendation, not a legal requirement. A pregnant woman can decline the test. If she does, the clinical team will typically treat her as if she might be carrying GBS and offer antibiotics during labor based on that assumption.
This is a decision worth discussing with a doctor or midwife. The conversation should cover the risks of GBS to the newborn, the benefits and limits of antibiotics during labor, and any personal concerns about testing or treatment.
It is also reasonable to ask which test your hospital or clinic uses, how long results take, and what happens if you go into labor before a result is available. These are practical questions that affect what care you receive.
What Are the Limits of GBS Testing?
No test is perfect, and GBS screening is no exception. A negative result near the due date does not guarantee a woman is not carrying the bacteria at delivery. Carriage can change in the days or weeks between testing and labor.
A positive result does not guarantee the baby will be exposed or infected. Many babies are exposed to GBS without becoming ill. The bacteria cause disease in only a small fraction of exposed newborns.
Antibiotics during labor are not risk-free. They can affect the mother’s microbiome and, in some cases, the newborn’s. They can also cause allergic reactions. The decision to use them weighs a small but serious risk to the baby against the risks and side effects of antibiotics.
This is why the test is a tool, not a verdict. It informs a decision that a patient and her clinical team make together, based on her specific situation.
Frequently Asked Questions
What is the group B strep NAAT test?
It is a molecular test that detects the DNA of group B strep bacteria from a swab of the vagina and rectum. It can give results faster than a traditional culture, which requires growing the bacteria in a lab.
Is group B strep NAAT the same as a culture?
No, they are different methods. A culture grows the bacteria and can show which antibiotics work against it, while a NAAT detects bacterial DNA and does not provide antibiotic-sensitivity information.
When should the group B strep test be done in pregnancy?
Screening is typically done between 36 and 37 weeks and 6 days of pregnancy. Testing closer to delivery gives a more accurate picture of whether the bacteria are present at the time of birth.
Does a positive group B strep test mean my baby will get sick?
No. Most babies born to mothers who carry group B strep are healthy. A positive result means antibiotics may be offered during labor to lower the small risk of early-onset infection in the newborn.
