Heredity is the number one contributing factor causing varicose and spider veins.
Women are more likely to suffer from abnormal leg veins. Up to 50% of American women may be affected. Hormonal factors including puberty, pregnancy, menopause, the use of birth control pills, estrogen, and progesterone affect the disease. It is very common for pregnant women to develop varicose veins during the first trimester. Pregnancy causes increases in hormone levels and blood volume which in turn cause veins to enlarge. In addition, the enlarged uterus causes increased pressure on the veins. Varicose veins due to pregnancy often improve within 3 months after delivery. However, with successive pregnancies, abnormal veins are more likely to remain. Other predisposing factors include aging, standing occupations, obesity and leg injury.