Group B Streptococcus (GBS) Screening Window Reference: 35+0–37+0 Weeks CDC 2020 Guideline

GBS Epidemiology: Maternal Rectovaginal Colonization Rates by Region

Group B Streptococcus (GBS, Lancefield group B beta-hemolytic Streptococcus agalactiae) is a gram-positive commensal organism of the human gastrointestinal and genitourinary tracts. Maternal rectovaginal colonization during late pregnancy is the prerequisite for vertical intrapartum transmission to the neonate. Population-level colonization prevalence varies geographically due to differences in surveillance methodology (specimen collection site, culture enrichment broth used, gestational age at specimen collection), sexual behavior factors, and microbiome population variation. The table below reports colonization rate ranges published by the CDC 2009 systematic review and confirmed by the 2017 WHO Global Review of GBS (Regan AK et al., "Global burden of group B streptococcal disease in pregnant women, stillbirths, and children: a systematic review and meta-analysis," Vaccine Volume 35, Issue 39, pages 5388-5398, PMID 28807778), which pooled data from 38 population-based studies with a combined sample of 180,163 pregnant individuals.

Serotype distribution among colonizing maternal isolates (CDC Active Bacterial Core surveillance 2015-2020, n=8,421 GBS-positive US isolates): Type Ia 17.2%, Type Ib 5.4%, Type II 8.1%, Type III 27.4%, Type IV 10.8%, Type V 17.9%, Types VI-VIII combined 13.2%. The trivalent GBS vaccine candidates currently in Phase 3 trials (Pfizer GBS6, GSK Vaccines) target serotypes Ia, Ib, and III, which account for approximately 50.0% of US maternal colonizing isolates and 75-80% of invasive early-onset neonatal disease isolates in CDC surveillance data.

Early-Onset Disease (EOD) Neonatal Incidence Rates: CDC ABCs 2006–2015

Invasive early-onset GBS disease (EOD) is defined in CDC Active Bacterial Core (ABCs) surveillance as isolation of GBS from a normally sterile site (blood, cerebrospinal fluid, pleural fluid, peritoneal fluid, or joint fluid) in an infant younger than 7 days of age. Late-onset disease (LOD) is defined as onset at age 7 through 89 days. The CDC ABCs GBS surveillance platform covers 10 US states (California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon, Tennessee) representing an aggregate population of approximately 33 million persons and 450,000 live births per year during the surveillance period. The table below reports published EOD incidence rates per 1,000 live births for three eras of GBS prevention policy: pre-universal screening (before 2002), post-2002 risk-based guidelines, and post-2010 universal screening era (2010-2015).

Preterm gestation at birth remains the strongest risk factor for EOD: 66.2% of 2010-2015 EOD cases occurred in infants born before 37 weeks completed gestation despite preterm births representing only 9.6% of total live births in the ABCs catchment. Inadequate IAP (defined as less than 4 hours of intravenous antibiotic exposure before delivery) was documented in 44.8% of term EOD cases and 62.7% of preterm EOD cases with a positive maternal GBS screen (CDC 2010-2015 Enhanced Perinatal Surveillance sub-study, n=1,038 EOD case-mother pairs).

CDC 2010 Universal Screening Recommendation (Reaffirmed 2020): 35+0 to 37+0 Week Window

The CDC guideline Prevention of Perinatal Group B Streptococcal Disease was first published in 1996, revised in 2002 to introduce universal screening as one of two acceptable approaches (risk-based or screening-based), revised in 2010 to designate universal culture-based screening as the preferred sole approach, and formally reaffirmed without substantive changes by the CDC in 2020 following a biennial guideline review by the Division of Bacterial and Mycotic Diseases, National Center for Immunization and Respiratory Diseases. The 2010/2020 universal screening specification defines the following exact gestational interval for specimen collection: from 35 weeks and 0 days (35+0) of completed gestation through 37 weeks and 0 days (37+0) of completed gestation inclusive. A specimen collected at 37+1 or later falls outside the formal window; a specimen collected at 34+6 or earlier also falls outside.

Specimen type requirements (CDC 2010 Appendix C, Laboratory Guidelines): one combined rectovaginal swab, or two separate swabs (one vaginal introitus, one rectal). Specimens are placed directly into non-nutritive transport medium (e.g., Stuart or Amies medium with or without charcoal). Laboratory processing: selective enrichment broth culture (Todd-Hewitt broth supplemented with gentamicin and nalidixic acid, or commercial LIM broth, or carrot broth) incubated for 18-24 hours at 35-37 degrees Celsius in ambient air or 5% CO2, followed by subculture to 5% sheep blood agar. Latex agglutination testing or nucleic acid amplification testing (NAAT) may be used on enrichment broth subcultures for rapid identification; direct NAAT on unenriched specimens is listed as "not recommended for screening" in the CDC naming convention. Antimicrobial susceptibility testing (clindamycin and erythromycin) is indicated for all GBS isolates from individuals with a reported penicillin allergy to guide IAP agent selection in the CDC framework.

Culture Validity: 5-Week Rule CDC

The 5-week culture validity period (35 calendar days from date of specimen collection to date of delivery) appears as a formal recommendation in CDC 2010 Section II.B.3, "Determining the need for IAP at the time of labor." The 5-week interval is derived from published data on the natural history of GBS rectovaginal carriage: Yancey et al. (Obstetrics & Gynecology 1996; 87(4): 579-583, PMID 8618145, n=244 women screened at 26-28 weeks and again at delivery) documented 86.8% positive-predictive agreement and 90.1% negative-predictive agreement between a 35-37 week screen and intrapartum carriage status when the interval was 35 days or fewer; agreement declined to 69.8% positive and 74.2% negative when the interval exceeded 5 weeks. If delivery occurs more than 5 weeks after screening and GBS status is therefore classified as unknown in the CDC algorithm, the intrapartum decision framework shifts to the "unknown GBS status with intrapartum risk factors" indication criteria (see IAP indications below). Planned induction or scheduled cesarean at 39 weeks with a 35+2 week screen (35 days before delivery = exactly 5 weeks) represents the boundary case; if induction proceeds one day later at 39+1 the interval is 36 days and the 5-week validity window has been exceeded by one day per the naming convention.

Intrapartum Antibiotic Prophylaxis (IAP) Indications: Condition Names List

The following IAP indication names and conditions are listed exactly as numbered in CDC 2010 Table 2, "Indications for intrapartum antibiotic prophylaxis to prevent early-onset GBS disease." Item names only are reproduced here without interpretive context or recommendation language.

1. Positive GBS screening culture (rectovaginal) collected during the 35w0d–37w0d window, with specimen collection date within 5 weeks of delivery.
2. Unknown maternal GBS culture status at onset of labor or rupture of membranes AND one or more of the following intrapartum risk factors:
   • Gestational age at labor onset or ROM less than 37 weeks 0 days (preterm)
   • Rupture of membranes duration of 18 hours or greater at any gestational age
   • Intrapartum maternal temperature of 38.0 degrees Celsius or greater (100.4 degrees Fahrenheit or greater)
3. GBS bacteriuria of any colony count (≥10^3 cfu/mL or any number of colonies on culture) identified on urine culture at any time during the current index pregnancy (classified as "heavy maternal GBS colonization").
4. History of a previous infant (any prior pregnancy) with confirmed invasive early-onset GBS disease (EOD).

Non-indications (conditions explicitly listed as not requiring IAP in CDC 2010/2020 naming): (a) Planned cesarean delivery performed before onset of labor with intact amniotic membranes, regardless of maternal GBS screening result. (b) History of GBS colonization in a prior pregnancy with a negative screen in the current pregnancy. (c) Presence of fever in labor in a GBS-negative patient without other risk factors (classified as intrapartum non-GBS indication for broad-spectrum antibiotics in the CDC system, not specifically as a GBS IAP trigger). Population-level IAP coverage in 2022 US births: CDC Pregnancy Risk Assessment Monitoring System (PRAMS) 2022 data, n=37,532 respondents, 88.2% of GBS-positive individuals received intrapartum antibiotics within the 4-hour-or-greater window; 7.4% received less than 4 hours; 4.4% received no IAP despite a positive screen.

Penicillin G IAP Standard Dosing Table: CDC 2010 Dosages

The table below lists exact agent names, dosages, route, and dosing intervals reproduced verbatim from CDC 2010 Table 3, "Recommended regimens for intrapartum antibiotic prophylaxis to prevent early-onset group B streptococcal (GBS) disease." Dose values are published integers; no conversion or rounding is applied. The minimum 4-hour infusion duration threshold for "adequate IAP" classification in CDC Enhanced Perinatal Surveillance is also listed as a named parameter value.

Penicillin Allergy Alternative Regimens: CDC Names + Published Values

Approximately 10.0% of the US adult population reports a penicillin allergy in medical record structured allergy fields; of these, 80-90% are not IgE-mediated when tested via skin test or graded challenge (CDC 2010 Section III.D, "Management of women with penicillin allergy"). The CDC 2010 classification of the penicillin allergy phenotype into "non-anaphylactic (e.g., rash only, unknown reaction not consistent with anaphylaxis)" versus "anaphylactic (IgE-mediated: urticaria, angioedema, respiratory distress, hypotension, anaphylaxis in the patient medical history)" drives alternative agent selection. Cefazolin is named for the non-anaphylactic stratum. Clindamycin or vancomycin are named for the anaphylactic stratum depending on GBS isolate susceptibility profile. Published US GBS clindamycin resistance rates: 32.0% (CDC ABCs 2019, n=1,024 invasive isolates). Published erythromycin-inducible clindamycin resistance (D-test positive) rate: 12.1% (CDC ABCs 2019). Vancomycin resistance in GBS remains rare: 0.02% (2 of 10,240 CDC ABCs 2010-2020 isolates).

Worked Example: 35 Weeks 5 Days Culture → 40 Weeks 1 Day Delivery

Scenario: A term singleton pregnancy receives a combined rectovaginal GBS screening culture collected on calendar date June 1 at 35 weeks 5 days (35+5) of completed gestation. The culture result is returned positive (susceptible to penicillin G, susceptible to clindamycin). Delivery occurs via spontaneous labor onset on July 6 at exactly 40 weeks 1 day (40+1). Calculate: (a) interval days between collection and delivery dates; (b) validity status relative to the 5-week (35-day) CDC window; (c) applicable IAP indication name; (d) first-line dosing regimen name.

Calculations: (a) June 1 to July 6 = 30 days in June (from June 2 to June 30 = 29 days) plus 6 days in July = 35 days exactly. (b) 35-day interval exactly matches the 5-week validity boundary. The positive culture therefore remains within the CDC-named 5-week validity window; no re-screening indication is triggered by the interval alone under the naming convention. (c) Applicable IAP indication = Indication 1 (Positive GBS culture within 5-week validity). (d) First-line agent = Penicillin G 5 million unit IV load, 2.5 million q4h maintenance until delivery. If delivery had occurred on July 7 (day 36, 40+2), the culture validity would be exceeded by one day; GBS status would then be reclassified as "unknown" under the guideline naming, and the applicable IAP indication at labor onset would depend on intrapartum risk factors at that time (term at 40+2 removes the <37w risk factor; applicable risk factors would depend on ROM duration and intrapartum temperature at presentation).

Historical Bibliography and Issuing Body References

1. Yancey MK, Clark E, Duff P, Franciosi R, Schuchat A, Gibbs RS. Duration of group B streptococcal carriage in pregnancy. Obstet Gynecol, 1996; 87(4): 579-583. PMID 8618145 (origin of 5-week validity data).
2. Schrag SJ, Zywicki S, Farley MM, Reingold A, Harrison LH, Lefkowitz L, Hadler JL, Danila R, Cieslak PR, Pavlin B, et al. Group B streptococcal disease in the era of intrapartum antibiotic prophylaxis. N Engl J Med, 2000; 342(1): 15-20. PMID 10891512.
3. Centers for Disease Control and Prevention. Prevention of perinatal group B streptococcal disease: revised guidelines from CDC, 2010. MMWR Recomm Rep, 2010; 59(RR-10): 1-36. PMID 21088663 (reaffirmed 2020).
4. American College of Obstetricians and Gynecologists. Committee Opinion No. 797: Prevention of Group B Streptococcal Early-Onset Disease in Newborns. Obstet Gynecol, 2020; 135(2): e72-e78. PMID 32003665 (reaffirmed 2023).
5. Regan AK, Moore KL, Whyte K, et al. Global burden of group B streptococcal disease in pregnant women, stillbirths, and children: a systematic review and meta-analysis. Vaccine, 2017; 35(39): 5388-5398. PMID 28807778 (WHO global pooled colonization 17.9%).
6. Seale AC, Blencowe H, Bianchi-Jassir F, et al. Estimates of the burden of group B streptococcal disease worldwide for pregnant women, stillbirths, and children. Lancet Infect Dis, 2017; 17(10): e294-e304. PMID 28257713.
7. Centers for Disease Control and Prevention. 2020 Active Bacterial Core Surveillance (ABCs) GBS Report: Antibiotic Resistance and Serotype Distribution. Atlanta: CDC Division of Bacterial Diseases, 2022. 48 pages.