I've seen a fairly big difference on most occasions.
I'm sure many remember the Paulson turf horse who had a win and 2nd at big odds after she returned to the races in foal. I think her name was Steal Buns or Buns of Steal or something like that.
There is a filly racing now at PID named Modern Verve that is pregnant. I actually thought about claiming her for 5K when she was entered on opening day - she dominated a 12 horse field at 16/1 odds in her return and I thought the race was a lot faster than the figure it got.
However, she was announced as in foal before the races. I didn't want to spend the money for her once I found that out. Turned out to be a mistake.
