Flying shouldn't cause any problems in your first two trimesters. Plan your schedule so that you're not rushed when arriving at the airport, and plan connecting flights so that you have ample time to get from one gate to another without being stressed. Book an aisle seat so that you have a little more room and can easily get to the bathroom as needed. Walk up and down the aisle every hour or so to promote circulation in your legs. While sitting, flex your feet toward your face and make circles with your feet. Wearing support hose also stimulates circulation in your legs when you have to sit for long periods of time. Drink lots of water or juice to stay well hydrated.
Travel by boat, particularly if it's a large cruise ship, also should pose no particular problems in the first two trimesters. And most cruise ships have medical personnel aboard should you need assistance. If you're sensitive to motion, you might want to take medication to prevent motion sickness; ask your health care provider what would be safe to take during pregnancy. You can also wear the anti-nausea acupressure wristbands that are available over-the-counter at your pharmacy.
Obviously, the farther you venture from home during pregnancy, the greater the risk becomes if you develop any complications while away. The risk is that of being away from your personal health care provider, who has your medical records and knows your history. If you do travel far from home, you can plan ahead by being sure there are good sources of medical care at your destination. Take a record of your pregnancy health care, including tests you've had done, medications you're taking, your blood type, and any other information that might be helpful when you're out of town. Take your health care provider's name and contact information as well. If you have to travel out of the country, it is important to take copies of your prescriptions for medications, in case your medications become lost. Be sure your immunizations are up to date before planning a trip to countries where vaccinations are necessary, for some vaccines may not be safe to update during pregnancy.
Be aware that changes in climate or altitude and types of food could cause you more discomfort when you're pregnant. Limit exertion for a couple days after your arrival at your destination, particularly if the climate is hot or the altitude is high; this will allow your body to adjust to these changes.
In addition to the above considerations, always consult your health care provider before planning a trip, particularly if you'll be experiencing changes in altitude. She or he may be able to give you a medical contact in the area of your destination, in case you'd need to see a physician while you're away from home.
Tips for Your Trip
If the travel you're planning is a vacation, consider the type of activity that would be best during pregnancy. For example, relaxing by the beach or at a lovely hotel would be far better than hiking in the mountains! A trip with a single destination would be better than a trip where you're frequently moving from one location to another. Don't plan too many activities in one day; pace yourself, for you will tire more quickly than when you're not pregnant. If you're traveling for business, arrive at your destination a day before the meeting or activity so that you can rest. Limit the length of your workdays, if possible, and take regular breaks.