I took the tablets from 32 weeks with both my pregnancies. I forgot a lot of times in my first pregnancy and my labour was long with a 15 minute pushing stage.
My second pregnancy I took them religiously and had a less than 5 minute pushing stage.

From my understanding raspberry leaf helps with making the contractions a lot more efficient and strengthens the uterine muscles making the pushing stage a lot shorter and easier on the body. Certainly seemed to work for me.