Talk

I think it depends on the condition of your cervix. There's no point in a sweep if your cervix isn't ready. Happy to be corrected but that's what my MW told me.

How would they determine that?

They can give you lots of sweeps but they won't work until you're starting anyway. The might just kick start things by a few hrs or maybe a day. I'd ask for one at 39.5. Right before you have to start preparing for your CS just in case you get lucky. I really don't see the point earlier.

I had a sweep the day before my due date and started labour the next day.

That latent phase of labour lasted four days for me- irregular contractions which stopped me from sleeping. By the time I actually moved to the next stage of labour I was absolutely exhausted from four nights of no longer than thirty mins between contractions.

I'll never know, but I just wonder if the sweep meant that it luck started labour earlier than I was ready and caused the long early stage

They would determine by looking at or feeling it.

You can always ask. I think the standard practice is they don't do them before 40 weeks, but if you are truly keen to avoid a c-section, you might find a midwife who is willing to do one for that reason. Usually they check your cervix and if it's starting to open already, they can do one. If it's not, then they won't be able to.

My midwife (or the others I pestered) wouldn't touch me until I was over 40 weeks, and I was HUUUUUUUUUUGE

(And when I did finally get one it didn't work anyway)

Absolutely reasonable to request this and I would certainly agree to it as a midwife, however they only really work if you were going to labour anyway

You can request but they won’t normally until after 40 weeks unless you are high risk/going to be induced in the next few days.

Skim read here.
I would be shocked if they didn’t offer you sweeps. Start asking now if you want a good few goes