I just went on a trip at 18/19 weeks that was 9-10 hours driving on Day 1 and about 4-5 on Day 2. (Actual travel time varied due to having a toddler in the car with us.) My midwife had no problems, but gave me a copy of my prenatal record just in case. She said that having it would mean I wouldn't need it.
As long as she's having a healthy, uncomplicated pregnancy and you're both willing to stop often for her to walk around, I don't see why her OB would have a problem with it. I can understand not traveling after 30 weeks because of the PTL risk, but before then should be fine in my non-medical opinion.
Semi-crunchy Mama to three happy, healthy girls - T(6/08), A (12/10) and B (12/10)