No one knows the exact cause of spider and varicose veins, but there are several factors that cause a person to be more likely to develop them. Heredity, or being born with weak vein valves, is the greatest factor. Hormones also play a role. The hormonal changes that occur during puberty, pregnancy, and menopause, as well as taking estrogen, progesterone, and birth control pills can cause a woman to develop varicose veins or spider veins. During pregnancy, besides the increases in hormone levels, there also is a great increase in the volume of blood in the body that can cause veins to enlarge. The enlarged uterus also puts more pressure on the veins. (Within 3 months after delivery, varicose veins usually improve. However, more abnormal veins are likely to develop and remain after additional pregnancies.)
Other factors that weaken vein valves and that may cause varicose or spider veins include aging, obesity, leg injury, and prolonged standing, such as for long hours on the job. Spider veins on the cheeks or nose of a fair-skinned person may occur from sun exposure.