I'm 36+ with #3 and desperately holding my legs closed for (fingers crossed) at least 2 weeks! My #2 was born at 37 weeks on the nose. She was 22 inches long and only weighed 6 lbs 12 ozs. There wasn't an ounce of fat on her. She was fine, high apgar, breathed well, nursed like a pro. But our first 4 weeks were horrible. She startled at the drop of a hat and her response was really exaggerated. she got worked up really easily and took forever to calm. I spent many, many a 3am on the couch with her crying while she screamed. The pediatrician said that sort of 'under-development' in neuropathways is seen often in babies born at 37 weeks. For that, I am resting with my legs tightly crossed trying to hold my fully engaged 36 weeker in so that it won't come for at least a couple of weeks.
My second daughter was born at 37 weeks brought on due to IUGR. She stopped growing and developing at 36 weeks due to the placenta failing. She was 6lb 1oz and I had no trouble with her. She was colicy so cried more than my first but that's about it. I highly think all babies are different and maybe if your lo was full term she may have still been like it. My 41 + 4 was the best baby ever and my friends 39+3 was like your lo and still is at 7 years old.
I think about that often, whether my #2 might have just been like that. We didn't IUGR, my water simply broke on it's own. My 1st born at 42+5 was the most amazing baby I have ever met. When she was 6 weeks old I took her to my GP as she had not yet cried (except when she was born). As it turns out, she was fine, just happy. Thinking a lot about how she may just have been easily stimulated and tending that way by personality, I have still decided that just in case the cause was a slightly early birth, I still plan to hold my knees together in the hopes that my sleepless nights are just a little less sleepless! Whatever you decide, I hope everything works out super well for you, and in the end, I don't think baby will come until baby is ready!