With your new, larger body, flying while pregnant (and really any travel during pregnancy) can be uncomfortable, stressful, and exhausting. There are a few tricks you can use to make the flight more comfortable.
First of all, no matter what happens, try to remain calm when you're traveling. Travel can be stressful under the best circumstances, and often, negative things happen. Whether you get deplaned and have to start over, your flight is delayed or canceled, other passengers or airplane crew or rude, or anything goes wrong, you just need to relax.
Ignore everything going on around you and keep breathing. Your baby can definitely feel your stress, so you need to focus on what's positive and happy.
The humidity levels are really low on planes, and combined with the high altitude and your increased need for hydration, it's much easier to get dehydrated when you're flying while pregnant. I take a refillable water bottle and fill it in the airport water fountain before getting on the flight, and I make sure to drink the entire thing on the flight, in addition to ordering water during the in-flight. Water will hydrate you much faster than a juice or soda; and of course, avoid caffeine and alcohol, which are even more dehydrating when at high altitude.
Make sure to get up every 30 minutes or so and walk around the plane. You're more likely to develop leg swelling and even deep vein thrombosis when you're flying while pregnant, so it's important to stretch. You can also point your toes and flex your feet back and forth to keep everything moving in your legs. There are compression socks and tights that you might consider wearing to avoid the swelling that can happen from the increased elevation.
Bring a pillow and a small blanket, even though it means taking up more space in your carry-on. Lower back pain can be exacerbated while flying, so a pillow stuffed by your back can make all the difference in the world. And, you'll be more comfortable if you can regulate the temperature yourself, instead of depending on the pilot to decide how cool it should be. I also dress in layers so I have more control over how warm I am.
The safest time to fly during pregnancy is during your second trimester. You're past many of the risks of the first trimester and not yet in the real danger zone for early delivery. Most doctors advise not to fly after your eighth month of pregnancy, and if you have a high risk pregnancy or are expecting twins, it might be even sooner. It's important to talk to your midwife or doctor before you start planning the trip to see if it's advisable.
Some airlines require a doctor's note when you're past 6 months. If you're carrying a really large lump (especially if you're smaller and the bump looks comparatively large on you), you should expect that those airlines probably won't believe you're only 5 months pregnant, so get a note just in case.
There are different rules when you're flying with a newborn, and some that are regulated by the airline industry, so make sure to check with them when you start planning your trip.
You can get an airplane-safe car seat if you fly often and know you're going to prefer having a seat for your baby. This is especially helpful on overseas or other long-haul trips. If you're taking a shorter flight, you can hold the baby in your arms and avoid the expense of an extra ticket.
Some families opt to bring the car seat and base and simply have it checked at the gate, so as to avoid some of the rough handling from the baggage loaders. You could also check your stroller there so it can be used in the airport. Others prefer to get rid of the things in their hands and just tuck the baby into a carrying wrap right at check-in.
Everyone's big concern on the plane is whether the baby is going to cry the entire flight or not. And, really, as a mom, it's hard to know for sure. You can certainly do your best to bring everything that might make the baby calmer but sometimes it's out of your control.
One thing you can do is to nurse on the ascent and descent to relieve the pressure on the baby's ears. If you are bottle-feeding, give him/her a bottle instead, and if they just aren't hungry, a pacifier would do the trick. The sucking motion often helps. You can also try to arrange the schedule so the baby will be tired and able to sleep for as much of the flight as possible.
If you're on a long flight, you should definitely plan on having to change the baby in the plane's bathroom. Most planes have a changing table in the bathroom, but you could always ask the flight attendant for assistance in getting the baby situated properly.
Flying while pregnant and traveling with a baby doesn't have to be stressful, even if you're both at the same time, and even if you are alone. People will likely offer you help when it looks like you need it, and much of the experience is in your hands. Try to relax and enjoy, and perhaps focus on the destination.