I had two. The first, I had been eating very healthily all throughout my pregnancy (avocados and pineapple were the things I ate regularly), but all of a sudden, I wanted mint chocolate chip ice cream for some reason. Around 7:30 p.m., I started having contractions. I timed them with a nifty contraction-timer on the computer, and messaged my midwife with my times. We got in the car around 11:30 p.m. or so, and arrived at the birth center a bit before 1 a.m. I had chosen it because I wanted a waterbirth, and I labored in the tub. Up until that point, I hadn't had any contractions at all-- no Braxton-Hicks, although some other ladies on my birth board had been experiencing BH for a week or two prior to actual delivery. It was a long time before my water broke (in the tub). Eventually, I had to get up and go to the birthing chair, because he was presenting with his arms up. (She knew how he was presenting, even without scans and stuff. She also mentioned about previous babies in her care who were readying themselves in a breech position who she had successfully compelled to flip and reposition themselves via rebozo techniques.) I tried a ball and a couple of styles of chair before he was born, around 5:45 a.m. It took a while afterwards to deliver the placenta, but those contractions weren't anything like the delivery itself-- that was a walk in the park. It was nice that when it was over, it was over-- and I got to concentrate 100% on my little guy.
When I had his little brother, I didn't take my contractions very seriously. I started noticing them around 9:45 a.m. The pest control guy came, and I'm in labor and wandering around talking to him. I figured he'd wait until nightfall before he was born. But things were getting more intense, and I may have had some spotting. So I called DH to come home from work around 10:30, just so we could make the drive to the birth center, because I knew I could handle it then--- but I wasn't so sure I could handle it in a few hours. So he got home a bit after 11 a.m., we made the drive, and we got there a bit after 1 p.m. I felt like I had to go to the bathroom, so I went and labored on the toilet for a while-- goshdarnit, that was the most comfortable toilet in the world! --and they finally coaxed me into the birth tub. (I knew in my head that childbirth would trigger your "I-gotta-go" feelings, but I was still thinking I had a ton of labor left, so I didn't think much of it.) She was shocked that I was as far along as I was. Once I realized I had permission, and that it was okay to push, I cooperated with the contractions-- and he was born at 1:34, probably within 20 minutes of our arrival. I remember feeling like my water broke earlier with this one, since it happened right after getting in the tub, but perhaps it was just a matter of all the events being more condensed into a shorter timeframe.
So for me, for my first, the only early warning sign was "I want to eat mint chocolate chip ice cream". It was me unconsciously starting to feel the early signs of labor, and wanting to eat something cold and minty and gentle to settle my stomach, before I actually consciously felt the contractions. But only an hour passed between "please go to the store, dear" and "oh, wait, these must be contractions" and there were about 10-11 hours between "oh, these must be contractions" and actual birth. For my second, I didn't have any early warning signs that I can think of-- I was consciously thinking, "I'm sure he won't be born for another 12 hours!" when, in fact, we went from first contractions to birth in under four hours.
I was also told that it makes a difference as to how many pregnancies that you've had-- that your subsequent labors tend to go much more quickly than your first. I don't know if that's true or not; it's just what my midwife told me.
Although I'd experienced a lot of fatigue during the first trimester, the rest of the pregnancy went along just fine. I worked until a week or two of my EDD-- I'd expected to work more, but I was more tired than I realized. I'd planned on taking one of my off days waiting for the baby to go catch the Tutankhamun exhibit in a nearby city, since it had been 30 years since the last US tour, but I ended up just lying on the couch... so that shows how unmotivated I was to do anything towards the very end!