I have 2 DS and the first being nearly 4 went through the stage came out the other side and the other just 2 is currently going through the stage at the moment.
I am convinced the behavioural problems start because we start talking to them about using the toilet / potty for doing poos / wees. At this age I think they don't always know they need to do a poo they just get a sensation go somewhere do it and almost say to themselves "oh I've done a poo" As most of us parents say "If you'd have told me we could have done it in your potty" they feel that they have done something wrong and that's why they then don't want to tell us.
My boys are quite regular with their poos either after breakfast or after tea so I tend to treat them with telly at these occasions and so my littlest sits on the potty whilst watching 1 episode of thomas - by which time he's done his poo, gets lots of praise and is now starting to ask for thomas after meaks , knowing that he has a time this is going to happen. Not that it will work for everyone but I think that they are really sensitive at this stage and they want to please us.
It takes a lot for them to think I have these feelings, possibly stomach ache, possibly being uncomfortable, knowing that that means I want to do a poo and then having the time to find us say I want the potty. Don't expect too much from them at this age. The more relaxed you are about it the better.
Ali, as for being pregnant and carrying a toddler kicking and screaming up the stairs with SPD maybe try keeping some change stuff downstairs. Putting the tv or music on for 5 mins to distract whilst changes or having his favourite toy to play with.
At this age I always believe there is always a reason for the behaviour that they are unable to tell us exactly what it is. work out why and you can often sort out the what!
I will get off my soap box now. Hope that is helpful.