Nobody has measured labor from 6 cm in America
American practice now calls 6 cm the start of active labor, and there is no American figure for how long the first stage takes from 6 cm.
This one is worth slowing down for, because it is a hole in the middle of the way American labor is now managed.
ACOG says active labor begins at 6 cm. That is the threshold your care will be judged against. So the obvious question is: starting from 6 cm, how long does the rest of the first stage usually take, and how long does it take for the slowest 1 in 20 women?
For an American woman, nobody has published the answer.
A systematic review searched for exactly this, across 37 studies and roughly 208,000 women. It found a single study anywhere in the world literature that used 6 cm as its starting point. That study was carried out in 13 hospitals in Nigeria and Uganda, where essentially no woman had an epidural. In it, first-time mothers took a median of 2.9 hours from 6 cm to full dilation, with a 95th centile of about 9 hours; women with one previous birth took a median of 2.2 hours with a 95th centile of 7.5; women with two or more took 2.4 and 7.4.
Those are real, carefully collected numbers. They are also numbers from a setting where almost nobody had an epidural, in a country with a different population and a different maternity system. In the American dataset, 84% of first-time mothers had an epidural. Moving those African figures onto an American labor ward would not be a small approximation; it would be an invention. This guide will not do it.
What does exist for American women past 6 cm is the centimeter-by-centimeter picture: for a first-time mother, going from 6 cm to 7 cm took a median of 0.6 hours with a 95th centile of 2.2 hours; 7 to 8 cm took 0.5 and 1.6; 8 to 9 cm took 0.5 and 1.4; 9 cm to full took 0.5 and 1.8. You cannot add those up to get a total. Adding medians does not produce a median, and adding 95th centiles does not produce a 95th centile.
And the two big bodies do not even start in the same place. WHO defines the active first stage as starting at 5 cm; ACOG defines it as starting at 6 cm. They are not disagreeing about biology. They have drawn a line at different points on the same curve. WHO also notes that Labour may not naturally accelerate until a cervical dilatation threshold of 5 cm is reached.
ACOG makes a similar point: Although this reflects an average starting point, there may be a range of dilation between 4 cm and 6 cm at which the rate of cervical change rapidly increases. However, standards of active-phase management and active-phase arrest should not be applied until at least 6 cm dilation.
So you may be given a threshold, 6 cm, that comes with no matching duration data for anyone like you. That is not a reason to distrust your team. It is a reason to ask what they are actually comparing you to.
"Is that from a study of women like me, with an epidural, in the United States?"
"How much time would you normally give this before we talk about doing something?"