I Got a Seroma After Flying at 18 Days Post Op. Do You Think This Was Due to the Pressure?

The seroma was also likely due to sitting in the cramped position for so long.  Use a compression garment, and follow your doctors advice.  This is another reason to have a Board Certified Plastic Surgeon from your area to manage your care. 

Drain removal at 14 days is a very "standard" length of time, but presumably your drain was taken out only after the drainage had tapered to less than 25cc or so per 24 hours, not just removed after a certain length of time. If your skin flaps had securely healed to the underlying abdominal wall layers, then the drain removal would have left nothing but a thin tract for any fluid to accumulate in. So, unless the drain had clogged while serum was still being produced (let's say at about 35cc per day), or was simply removed prematurely, 4 days of undrained fluid at 35cc per day = 140cc, almost exactly the amount your doctor aspirated.

Perhaps the drain being out and your taking a trip both conspired to permitting you a bit more activity, and instead of gradually reabsorbing any remaining fluid, your subcutaneous (not-yet-healed-shut) cavity accumulated the seroma. Aspiration is appropriate treatment, and will need to be repeated several times until the amount diminishes without reaccumulation. You should limit strenuous activities, as well as wear your compression binder full time (except for gently showering), until completely healed.

If the aspirated seroma amounts remain high, your surgeon may recommend reinsertion of another drain, which will keep the cavity continuously empty, allowing the tissues to seal and heal, or allowing sclerotherapy (with Tetracycline, for example) to stimulate closure of the pseudobursa cavity.

Flying and pressure changes are not the culprits; activity and incomplete healing with accumulation of seroma is a rather common occurrence we all see from time to time. Travel if you must, but try to keep your activity levels low, and your binder on! Best wishes!

I don't believe a change in pressure would cause a seroma.  It could have been from a lot of moving around or just because your body was still making a lot of fluid.  Sometimes, the drains get clotted and seem like the output is low while your body is still making a considerable amount of fluid.  With the amount you had, you will probably need a few aspirations.  I would also continue with good compression, a low salt diet and limited activity.  Good luck.