I have this long blue prominent vein that runs down my thigh. Could it be a varicose vein? (Photo)

The veins on your photo do
not look pathological: they are straight and, therefore, are not stretched
along, which means they are not stretched across either. Once your
weight is back to baseline and / or if your legs bother you, you might want to
have a formal evaluation performed by a physician specialized in venous
circulation. Hope it helps. Good luck!

The vein you are describing and referring to in the photo appears to be a reticular vein, which, although not tortuous in nature, has become more prominent during and subsequent to your pregnancy. This is not uncommon, where the hormones of pregnancy adversely affect veins in the leg in those who are predisposed to developing vein conditions due to family history. Other factors such as increased weight and blood volume during pregnancy can further, temporarily, adversely affect the lower extremity veins.The hormones can weaken both the vein wall (estrogen) and the valves (progesterone) in the vein, which leads to venous insufficiency. These "leaky valves" are the cause of veins ranging from telangiectasias (spider veins), to the larger more prominent, darker "blue veins" (reticular veins), to the bulging, tortuous (varicose) veins. All these conditions are due to the same underlying condition- valve dysfunction- and are a matter of severity. You should seek evaluation by an expert in vein conditions and become educated about your condition, including treatment options if necessary.

Hello, From the photos alone, a proper diagnosis cannot be adequately made. I agree that  it is more in keeping with a Reticular vein. During pregnancy, many women do see an increased appearance of leg veins. They can be asymptomatic. I would recommend having a DUPLEX ULTRASOUND, A NON-INVASIVE TEST to fully map out your leg veins and to ensure that you don't have any underlying venous insufficiency. Support stockings, 20-30 mmHg are always recommended.

Thank you for your concern.
Evaluating your photo, you do have a varicose vein. Varicose veins occur when
the muscles that surround our veins contract, due to improper functioning of
the valves. If these valves do not function correctly, a certain amount of
blood will cycle back within the veins, which create overloading veins that
occur especially in the legs due to gravity. As a result, veins that are close to
the outer layers of the skin appear larger and in vivid color due to the additional
blood. To smooth large veins, Sclerotherapy is used to reduce the appearance of
large veins. 

What you are describing is classic for venous insufficiency. Typically gets worse during the pregnancy state where it is first noticed in women of child bearing age. It is worse at the end of the day because of venous pooling of blood. It is best in the morning because you were recumbent all night. It is better when you elevate the leg because ithelps drain venous blood in the correct direction, i.e. towards the heart. Leakage or venou spooling would pool blood away from the heart, toward the feet, which occurs when gravit helps blood go down towards the lowest point of the body where hydrostatic pressure is highest. Classic signs of venous insufficiency are - aching and pain in the legs, heavy legs, tired legs, itching and burning (not common), fatigue of the legs, cramping, throbbing and restless legs, and of course the big one - leg swelling or ankle swelling. Read the ebooks provided on the link below.Our Los Angeles offices - Santa Monica Vein CenterOur offices in Western NY - Williamsville and Clarence - VeinsVeinsVeins.com

This is a reticular vein.  It's larger than a spider vein but smaller than a varicose vein.  These reticular veins are just under the skin and usually do not cause any medical problems.  They can be treated with sclerotherapy. 

The picture looks like a blue (reticular) vein just below the skin.  The saphenous vein runs in the same vicinity but deeper in the skin.  This vein should cause you no harm but if it bothers you, most likely it can be treated by sclerotherapy.  Before doing this you should see a vein specialist for a full venous evaluation.