The anal swab during pregnancy is part of a routine screening test for a bacterium called Group B Streptococcus, or GBS. It happens during the 36th or 37th week of pregnancy, and the reason they swab the rectum (not just the vagina) is that GBS lives in the gastrointestinal tract before it colonizes the vagina. Swabbing both areas catches carriers who would be missed with a vaginal swab alone.
What GBS Is and Why It Matters
Group B Strep is a common bacterium that lives harmlessly in about 25% of adults, typically in the intestines, vagina, or rectum. It’s not a sexually transmitted infection, and it’s completely different from the Group A Strep that causes strep throat. Most people who carry GBS never feel sick or have any symptoms at all.
The concern is specific to childbirth. GBS can pass from mother to baby during labor and delivery. Among babies born to carriers who don’t receive treatment during labor, roughly 1 to 2 out of every 100 exposed babies develop a serious infection in their first days of life. That’s a small percentage, but the consequences can be severe: bloodstream infections, pneumonia, and meningitis. About 20 to 30% of babies who survive GBS meningitis have lasting problems like hearing loss, seizures, or delays in learning to move, speak, or think. A small number of babies with early-onset GBS disease die even with immediate treatment.
Why They Swab the Rectum, Not Just the Vagina
This is the part that catches most people off guard. The gastrointestinal tract is the primary reservoir for GBS. The bacteria colonize the gut first, and from there they can spread to the vagina, where they pose a risk during delivery. Research published in the American Journal of Obstetrics & Gynecology found that rectal GBS colonization was the single strongest predictor of vaginal colonization, with carriers in the rectum being about 14 times more likely to also have GBS in the vagina.
If providers only swabbed the vagina, they’d miss a significant number of women who carry GBS rectally and could have vaginal colonization by the time labor begins. The combined vaginal-rectal swab is the most reliable way to identify carriers.
What the Test Feels Like
The test itself is quick and painless. A cotton-tipped swab is gently wiped across the lower vagina and then the outer rectum. It takes a few seconds. The swab doesn’t go deep into the rectum. Many providers now offer you the option of doing the swabbing yourself, which some people find more comfortable. The swab is sent to a lab for culture, and results typically come back within a day or two.
When and How Often You’re Screened
Current guidelines from ACOG recommend universal screening between 36 weeks 0 days and 37 weeks 6 days of pregnancy. This timing is intentional: the results remain valid for about five weeks, which covers births up to at least 41 weeks. GBS colonization can come and go, so a negative result from earlier in pregnancy doesn’t reliably predict your status at delivery. That’s why the test is repeated with every pregnancy, even if you tested negative before.
Every pregnant person gets screened regardless of whether they’re planning a vaginal delivery or a cesarean, with two exceptions: those who’ve already had GBS found in their urine during the current pregnancy, or those who previously had a baby with a GBS infection. In both of those cases, treatment during labor is automatic and no swab is needed.
What Happens If You Test Positive
A positive result doesn’t mean anything is wrong with you. It simply means you’re a carrier. The practical impact is straightforward: when you go into labor, you’ll receive antibiotics through an IV. The antibiotics work quickly to reduce the amount of GBS present in the birth canal, dramatically lowering the chance of transmission to your baby. The earlier in labor the antibiotics are started, the more effective they are.
If your water breaks more than 18 hours before delivery, the risk of GBS infection for the baby increases about fourfold, because bacteria have more time to reach the baby. This is one reason providers monitor the time between membrane rupture and birth closely in GBS-positive patients.
What GBS Infection Looks Like in Newborns
Early-onset GBS disease appears within the first week of life, most often in the first day or two. Signs include fever, poor feeding, difficulty breathing, unusual sleepiness or irritability, and skin that looks bluish or pale. Meningitis may cause seizures or a bulging soft spot on the baby’s head. Late-onset disease can develop anytime from one week to about three months of age, though it’s less directly linked to colonization at birth.
These infections are rare precisely because screening and treatment work. The combination of identifying carriers at 36 to 37 weeks and giving antibiotics during labor has dramatically reduced early-onset GBS disease over the past few decades.