There is an interesting theory re poo creating issues for toilet training. My (ex) shrink believes that toddlers see poo as being part of themselves, and don't want to let it go. This is why when toilet training they are often fine and easy to train with pees but either withold poos or only do them in pants or a nappy. And listening to other mums at nursery this seems to be pretty common, though I didn't have that issue with DD1 - in fact as I said yesterday her constipation finally cleared up when she was toilet trained.
Where it is relevant to me though is LO's constipation - she needs to poo, she knows she has to, doesn't have any reason not to, but holds it in. Could it be because she sees her poo as part of her?
Also, around 18mo DD1 started to hate having her nappy changed - really dislike it, and each nappy change was a physical battle, which was great fun when recovering from the c section. I suppose that would fit in with the theory too?
This is the age where they start to see things as belonging to them. One of LO's words is "mine" and she gets v angry with DD1 for using what she perceives to be "her" beloengings. Unfortunately DD1 is still in that phase too which makes for lots of fun.
Freud had a lot to say on potty training too - called toddlerdom the Anal stage and of course managed to link it to sex. A lot of his theories have been discredited but they are interesting to read. I read up quite a bit on Freud because in France they are still very Freudian in their outlook, shown throughout the health and education systems, so I need to know what I am up against.
yw I have also heard the going up and down stairs thing is a good sign toddlers are ready, and that there is no point in even trying before then.