I'm starting mine on 14th March when I'll be 34 weeks.
I think I could realistically stretch to 36 weeks but I'm lazy and hate my job. I work in a supermarket so there's a lot of sitting or standing for long periods, and stretching. I'm finding I get backache and pelvic pain already when I've been at work for a few hours. I only work 3 days a week though so it's not too bad.
Also, the first week of my maternity is James' first week of half term (he has 3 weeks off). It will be nice to spend 3 weeks with him with no work or school rather than having to put him in the holiday club.
If I worked in an office I think I could stay a bit later as in that environment it's easier to take it easier and move about, and take breaks when you need to.
Looking forward to having a chance to get the house in order, and then relax and spend time with James before the chaos of a newborn starts!
I don't see why you couldn't stay till 37 weeks if you're feeling ok. You can change the date as long as you give 28 days notice. If your baby arrives early your maternity leave would start automatically.