I'm not sure as I am a ftm. A lot of women swear that the tea helped them to dilate and bring on contractions.
I really wish doctors would actually research it before they recommend it for labor. It does not start labor at all. It strengthens and tones the uterus. You can drink it from the very beginning of pregnancy. It's highly recommend for women with a history of miscarriage.
The only thing is does for labor is make your uterus stronger and therefor causing the contractions to be more effective when labor does start.