My first real contractions (as in "That was not a Braxton-Hicks!") started around 2:30 am. I couldn't sleep through them, but it was largely out of anticipation and not pain at that point. I finally had a glass of wine and slept from 5:30-7:30, when I woke up because they were way too painful to get more sleep. I didn't want to be the dork who called the midwife for a false alarm, but I finally decided they were regular enough and strong enough when they were averaging about five minutes apart and a little under a minute each - which was at 10:00 am. By the time the midwives showed up at 11:00 am, I was on my ball breathing "ooooooopen" through each one - but I don't think I was really at the can't-do-anything-else point yet. I will have to see a copy of my chart to know where things went from there, time-wise, because I lost track after that. I don't know when I was 4cm because I didn't have an internal check until I was 6/7, which was many hours later... and my water didn't break until about 15 minutes into my hour of pushing.
My little one was born at 9:42 pm. If you count from the first contractions, that's 19 hours. If you count from the time things were intense, I'd guess we'd be at more like 10 hours. Honestly, I'm not that worried about having an exact number, 'cause all I know is it was way less than my mom's 36 hours with me!
