Hi mommy, I'll answer some of your questions in parts and hopefully it will ease your worries a little bit.
1a/b) If you are in labour and you know that you want an epidural, you can actually ask for one as soon as you get there. Some choose to wait till the labour pains start actively but from what I heard, you can ask for it whenever you want.
2a) Epidural is administered via a catheter. It is left in the epidural space, medication can be given into the epidural space to treat labor pain for as long as is needed. The end of the catheter that is outside your back will be attached to a device that will continuously deliver medication into the epidural space to maintain continuous pain relief throughout your labor and delivery.
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Badass Momma