The Robson Ten Group Classification for any two or three states · 2016–2024 · observed rates, not predictions
A group can raise a state’s overall rate two ways: by being larger than it is elsewhere, or by having a higher cesarean rate inside it. The contribution is the group’s size multiplied by its cesarean rate, in percentage points of the overall rate, so the column sums to the overall rate. The difference column is what that group adds to the gap between the two states.
Why this module uses a different source. The natality public use file carries no geography. Its User Guide is explicit: “Beginning with the 2005 data year, the public release micro-data natality file no longer includes geographic detail (e.g., state or county of birth).” The other three modules on this site are built from that microdata; this one is built from CDC WONDER, Natality 2016–2024 expanded, which carries the same items plus mother’s state of legal residence. The classification, the inclusion rule and the pregnancy rule are identical to the other three modules, and every group was checked against them before this page was published; the two agree to within 0.1 per cent on every group.
Population. Hospital births to residents of the 50 states and the District of Columbia, gestational age from the obstetric estimate valid at 17 to 47 weeks, 2016 to 2024. 32,291,352 pregnancies. Nothing is excluded for a missing characteristic; women whose presentation, previous cesarean or parity was not stated are carried as Unclassified rather than dropped.
The unit is the pregnancy, not the baby. A twin pregnancy is one record, taken at the delivery of the last-born, so the pregnancy counts as a cesarean if either baby was delivered abdominally. Quadruplet and higher-order sets are the one place this module cannot match the others exactly: WONDER collapses set order at “3rd or higher”, so the last-born of a quadruplet set cannot be isolated. That affects roughly 40 sets a year nationally, 0.008 per cent of group 8.
Suppression. CDC WONDER suppresses any count of one to nine births, and publishing such counts is prohibited. Suppressed cells are excluded from the totals shown here rather than imputed, so a figure carrying the ▲ mark is a lower bound, understated by at most nine births per suppressed cell. Across the pooled table 507 of 20,349 cells are suppressed, almost all of them vaginal births in rare groups in small states; they account for 0.003 per cent of deliveries. The by-year figures carry more suppression than the pooled ones, which is why the pooled table is the default.
Group 9 — a real limitation. Robson group 9 is transverse and oblique lie. The birth certificate offers only “Other” for presentation, which conflates abnormal lie with face and brow. The observed rate in group 9 is therefore lower than a pure abnormal-lie group would give.
Suggested citation. Centers for Disease Control and Prevention, National Center for Health Statistics. National Vital Statistics System, Natality on CDC WONDER Online Database. Data are from the Natality Records 2016–2024. Accessed 29 August 2026.