With my two singletons I never had a pregnancy complaint until a week or two before birth. And I carry low, in my pelvis, it just wasn't a big deal.
But I am experiencing what you're talking about. I just can't remember how soon it started. Definitely not 17 weeks, as at that point I didn't know it was twins and had no indication, but I'm going to say sometime in the last few weeks. Maybe 24? It just hasn't happened often.
The other day I got up at my daughter's gymnastics class and literally couldn't walk right. I called the dr because of the pinching pelvic pain and pressure! But I'm 29 weeks.
I don't know the experience of others but so far it hasn't been constant for me at all. It doesn't happen every time. And I'm pretty sure that the night before the gymnastics class one of my LO's turned head down (they've been all over).
Valerie, wife to Kevin, mother to Elena (4), Jonathan (2), and twins, Andrew and Benjamin (2/2010)