Assuming that both mother and baby are healthy, you can travel when your infant is two weeks of age. In some cases you can fly even sooner than that, but I would recommend holding off if it's not absolutely necessary.
There are a number of things to take into account when deciding to fly with a newborn. Here are the most important.
First, you are exposing the baby to possible infections on a crowded airplane. Colds and and the flu are transmitted easily in a closed airplane cabin with recirculated air, and those viruses are much more dangerous to a baby at a week or two of age than one who is four or six months old.
Breastfeeding transfers many protective antibodies to a baby, however, and may help make her less susceptible to infections.
Second, babies are notoriously unpredictable in the first few weeks, with irregular sleeping, feeding and crying times. While this can certainly be managed, it may be very draining to Mom and Dad to travel before a baby has "settled." Some babies do this by two weeks, others not until three months.
Third, Mom should be healthy and recovered from the delivery. Women are at greater risk of problems, such as blood clots in the legs after delivering a baby, and sitting for a long period on an airplane only increases this possibility. If you travel that early, you should wear support hose, drink plenty of fluids and get up frequently during the flight to walk and stretch.
If there were any problems or complications with the delivery, then airplane travel should be avoided until the baby's doctor gives approval. Premature babies, and babies who had respiratory or feeding problems in the first week, fall into this category.
Obviously, there are many factors to consider when making plans to travel with a newborn. A one- or two-hour flight is less problematic than a six- or seven-hour, transcontinental or transoceanic flight. I would recommend that the baby have her first visit with the pediatrician prior to traveling.
And if you need to travel while you're still pregnant, check out our tips on traveling during pregnancy.