The twelve groups every birth belongs to, counted out of 31.4 million US hospital births, 2016–2024.
The United States cesarean rate for these nine years is 31.9 in every 100 births. That figure is correct and it is nearly useless, because it is the average of a first labor starting on its own and a booked repeat operation and a breech baby and a set of twins, all added together. Almost nobody is the average.
In 2001 Michael Robson proposed a different way to look at it. Split every birth into ten groups using only what is known before the birth begins — how many babies the woman has had, whether she has had a cesarean before, one baby or more, how the baby is lying, how many weeks, and how labor starts. Two of the groups are split in half again, which makes twelve. Every birth falls into one group and one group only, and the groups never move: a woman is classified by what was known beforehand, never by how the birth turned out. The World Health Organization adopted the system, and it is now the standard way of comparing one hospital, or one country, with another.
It applies those twelve groups to 31,384,803 hospital pregnancies delivered from 22 weeks between 2016 and 2024, taken from the National Center for Health Statistics natality public use files — the anonymous record of every birth certificate filed in the country. Pick your group and you get its cesarean rate, how large the group is, and how much of the national rate it accounts for. You can then add age, BMI and the exact week inside the group; Robson treats those as overlays on the classification, never as part of it.
Nothing is modelled, fitted, smoothed or predicted. Every number is one count divided by another. The twelve groups add back to 31,384,803 and to every cesarean in it, with none left over.
Read the size of a group before you read its rate. Breech, twins and unusual lies all have very high cesarean rates and all are small, so they move the national figure hardly at all. A middling rate in a large group moves it enormously. That is the whole point of the system.
Groups 2b and 4b are defined by their ending — a cesarean performed before labor ever began — so their rate can only be 100 in 100, and only their size is worth reading. On United States birth certificates these two groups come out larger than they do almost anywhere else. Robson has written that misclassification on the certificate is a likely part of the reason. Read them with that in mind.
It is not a prediction about any one woman, and it cannot tell you what causes anything. If one group has more cesareans than another, this page cannot tell you whether that is because of something that was done or because of the reason it was done. Two items on the certificate — whether labor was induced, and whether labor was attempted before a cesarean — are tick boxes filled in differently from state to state, so read the split between “before labor” and “after labor began” as a good guide rather than an exact figure.
Nothing you type is sent anywhere. The whole tool runs inside this page, on your own device.
Every group also has its own page, which opens straight on that group's numbers — birthdata.app/robson-calc/group-1/ through group-10. Those are the links to cite, bookmark or send to a colleague.
Not sure which group you are in? The same data, reached through eight plain questions → — they work out your group for you.
1. Robson MS. Classification of caesarean sections. Fetal Matern Med Rev. 2001;12(1):23–39.
2. Robson M, Murphy M, Byrne F. Quality assurance: The 10-Group Classification System (Robson classification),
induction of labor, and cesarean delivery. Int J Gynaecol Obstet. 2015;131(Suppl 1):S23–S27.
doi:10.1016/j.ijgo.2015.04.026
3. Robson MS. The 10-Group Classification System — a new way of thinking. Am J Obstet Gynecol.
2018;219(1):1–4. doi:10.1016/j.ajog.2018.05.026
Remember: this is a count of what happened to women whose records matched your answers, not a prediction about you. It cannot know your health, your baby, your hospital or your doctor. Read the full notes again →