I provide women with birth labour support and find the best time to attend is when the labouring woman can no longer "talk through" the labour surges and they are about 3-4 minutes apart.
Attending before hand I often find this will keep the woman in her "left brain control centre" and this is often counter productive. I've gone in earlier and regretted it, as I often find it makes the labour longer.
Midwives will generally attend and review the situation and often they tell mom to go to bed and sleep, until things progress along a little further.
If it's a first time mom, often the excitement gets the better of her, and this can leave her lacking in reserves at the "other end".
Sleeping through the surges until she's well into her labour is really the best way for all.
And moms can do this, if they are not "entertaining the masses".
Attending around the 4 cm mark is usually around this time, when she can't talk through them, and they are about 3-4 minutes apart and about 45 seconds in length. Dad can easily time this and often just a short conversation on the phone helps me determine where the mother is.
Beforehand, it can really interfere with her getting into the right state of mind to allow the labour to process as it will.
Too early and it seems like the "party has started" but it really hasn't, not quite yet.
I find when mom is on her own, in her quiet, able to make her own sounds and no one is listening, yet knowing she's safe and everyone is ready to attend her...she will just relax herself right into Active Labour.
I'd love to share some of my birth stories and here's a few.... Kathy