Planning:
If you have a choice of when to fly, or are planning on taking a babymoon, try and plan the trip during your second trimester. You'll have more energy, be more comfortable, and have less risk of going into preterm labor.
Check your airlines guidelines for flying. If you decide to fly during your third trimester, you'll want to confirm that your airline allows this, and what they'll require. Some airlines do not let you fly past certain weeks. Some will allow you to fly up until your last week of pregnancy as long as you have consent from your doctor. Talk to your doctor ahead of time about the trip, and plan a doctor's appointment for within 72 hours of your departure date. Some airlines require doctor's permission within 72 hours of your departure for you to be allowed on the flight, so it is better safe than sorry. You'll need to have a signed form from the doctor for when you board. (For me, I did not need a specific form, I only needed the doctor to type up a permission form himself and sign it.)
While you're talking to your doctor about your trip, you may want to ask for recommendations of doctors or a practice to visit at your destination. In case problems arise while you are on your vacation, you'll want to have someone who knows your medical history and about your pregnancy thus far, as well as someone who will take your insurance. Your doctor may have a recommendation, or you may have to pursue finding a doctor on your own. It will be beneficial planning to research the nearest hospital to where you are staying as well. I say this not to cause you worry, actually to do the opposite. If you are like me, planning eases your nerves, and you'll feel better knowing that I'd something happens, you have a plan for what to do to protect you and your baby.
You may want to invest in travel insurance. I don't say this because my fiancé is a travel agent (though I have seen him help many a stranded client because they were smart enough to purchase the insurance). It is in your best interests to get the travel insurance in case you do get stranded or there are complications and you need to return home sooner than you planned. It will save you money and take some of the pressure off you if you get caught in a bad situation. A stress-free mommy is best for the baby.
Consider booking a first class seat. I flew coach at 30 weeks, and while it was not terrible, I didn't have a lot of room to stretch my feet. If you have the money, book first class. Your swelling feet, and achey back will thank you for it.
Also make sure to book yourself an aisle seat. You'll be grateful not to have to climb over anyone when you get up to go to the bathroom or walk around. It also gives you a little more space to stretch out and get comfortable.
Packing
When it comes to packing for your trip, you want to keep in mind your destination. Then, almost as important, you want to keep in mind your comfort. You want clothes that will be comfortable no matter what you do or where you go. If you're feeling anxious about looking cute on your trip, remember that accessories are your best friend. Whether it be some cute scarves (if you're picking a colder destination), or a cute tote that matches your sarong (for a beach destination), that will make your outfits feel cute, when you're self conscious about your big belly and changing body. Make sure you have comfortable shoes if you are going to be doing a lot of walking.
Don't forget to take any medications you might need. For me, this was Tums for my acid reflux and my prenatal vitamins.
Also have on hand your medical information, insurance cards, any paperwork that might be important to have in case you have to stop in at a doctor's office.
Make sure to pack snacks in your carry on bag. You never know if the food on the flight is going to be alright for you to eat or if it is going to make you feel sick. It's important for you to keep up your strength.
Flying
Dress comfortably! Sometimes planes are hot, sometimes cold. When you're pregnant, you never know how you are going to feel, so dress in easy to remove layers and comfortably.
Wear comfortable shoes that are easy to slip off. I know it sounds obvious, but I wore sneakers to be comfortable and ended up regretting it. On the flight back, my feet began to swell, and because of turbulence I couldn't walk around. It was incredibly awkward for me to slip off my sneakers and even harder trying to put them back on when we had landed.
Walk around as much as you can. For me it was enough just to get up and go to the bathroom every hour or so (pregnant woman bladder haha). This will boost circulation and help prevent swelling. Try to walk when you can, even if you don't have to go to the bathroom, you never know when they might turn on the seatbelt sign. Like I said before, we had turbulence on our returning flight and so we could not get out of our seats for most of our 9 hour flight; I regretted not getting up when I had the chance, and my swollen feet weren't happy either!
You may want to take something on your flight to help alleviate motion sickness. Ginger snaps or hard candy can help, or a bracelet that is designed to help. If you don't have either of these things and start to feel sick on the flight, avoid reading, set the fan to blow on your face to help you cool down, and apply some pressure to the inside of your wrists to help reduce sickness.
Make sure you wear your seatbelt low and under your belly. It will be more comfortable like that, and safer for you and baby.
Don't be afraid to ask the flight attendants for help. Don't take advantage of them, but if you need a drink or to go to the bathroom when the seat belt sign is on, make sure to ask. They are there to make everyone more comfortable, and will do what they can to help you out. Our flight attendants were very courteous to me, and often would offer me something to drink each time they went by to make sure I stayed well hydrated, and even allowed me to go to the bathroom once when the seatbelt sign was on.
Finally, have fun and relax! It is a different experience traveling when pregnant, but it is even more difficult traveling with a baby. It may be your last trip in awhile, so enjoy yourself!
