I went on a 6 hour car trip when I was 7 1/2 months for a one week holiday and it was fine - I hate travelling so I didn't enjoy it ever but it was no wrose than when I wasn't pregnant (Just had to stop to pee more). It's just sitting - hell, I sat in an office until I was over 8 months all day on a much more uncomfy chair than a car seat Unless there's something wrong that makes sitting unbearable or you are high risk there's no reason you can't go. Have fun, enjoy travelling without a baby while you can!