I've heard that Paleo + CrossFit = Mass Fertility.
We're in a similar boat...on track for a mid-December delivery and recent end of 1st Trimester checkup shows a well-developing and laid-back baby!
Some OB-GYNs will advise to keep HR below 140-150 BPM. This is simply a guideline and the concept being that HR elevation (from exercise) due to increased muscle activity will bias O2 intake towards muscle tissue and away from organs and baby.
Because of some troubles before, we're not taking any chances so my wife is wearing a HR monitor that alerts over 130 (the thinking that HR is on the way up). She initially had it set to alert at 140 but was then peaking near 160. She is also avoiding long WODs that might increase body temp too much.
She is subbing, on occasion, from http://crossfitmom.com/