Traveling in and of itself, while a great convenience and even a luxury, definitely exposes us to risks that we are not exposed to when we are stationary. These risks obviously increase along with the duration of the journey. Proper health and safety during long distance traveling is even more precarious for expectant mothers, since pregnancy makes them more vulnerable in general. If you are expecting, this does not mean that you should halt any travel plans you may have altogether; it simply means that you need to exercise more caution with regard to your well being throughout the course of your journey. There are certain things to look for when traveling by plane or by car.
While there are no time limits to traveling by car, don't assume that it is safer than traveling by plane; this is actually not so. Chances of injury and circulatory problems are often worse when traveling by car. That said, it may not always be a choice; airlines generally do not allow women to fly with them when they are in their third trimester. At any rate, if you are planning to travel via plane, the best time to do so is in the second trimester. This is because the pregnancy should be well established by now (chances of miscarriage are greatly diminished at this point) and generally the mother's health tends to be best during this period—energy levels are high, the sickness associated with first trimester has passed, and the swelling and discomfort of third trimester has yet to come. That said, you should still watch for the following:
Germs – Your immunity, as an expectant mother, is quite weak. The physiological reason for this is that if it were working optimally, your immune system would actually see the baby as a foreign entity and try to fight it (and in women who have hyperactive immune systems generally, this can often be the case). So, with lower immunity, you are far more susceptible to germs. Both airports and the planes themselves are saturated with germs, particularly during the winter season. Frequent handwashing is the best way to keep these germs out. Additionally, avoiding directing contact between your skin and shared surfaces (like doorknobs) is good practice (you can achieve this by using a paper towel as a barrier between your hand and the door when you are exiting a restroom, for example).
Hydration – You may not realize it because you're often “just sitting there” but in pregnancy, your body is hard at work. If you are diligent about increasing water consumption during exercise, then it follows that you should do the same for this different sort of work too. Adequate water intake is especially important during pregnancy as it is the best way to fend off urinary tract infections, which expectant mothers are highly susceptible to. Anything that diminishes the chance of infection during pregnancy is your friend, since you don't want to resort to antibiotics unless it's absolutely necessary. (That said, there are antibiotics which are safe to take during pregnancy, so if you do require them, you should not hesitate to use them.)
Thrombosis – This is actually one of the worst risks associated with either type of travel during pregnancy, but it can actually be more of a risk with car travel—particularly if the expectant mother is one the driving. Circulation is not necessarily at its best in an expectant mother to begin with, and weight is being pushed on vessels that normally have free access. Add to this the long periods of physical inactivity associated with travel, and circulation is further inhibited. The antidote for this is to stand, stretch, and walk about frequently (making rest stops often if you are traveling by car).
Radiation – While increased radiation is not a factor in traveling by car, airport scans are a source of unwanted radiation for the expectant mother. To avoid this, you can let the security personnel know that you would prefer to opt out of the scan, and to be examined physically instead.