Heredity is the number one factor causing varicose and spider veins. Women are more likely to suffer from varicose veins, especially because of pregnancy and hormonal factors. Pregnancy increases hormone levels and blood volume, which in turn causes veins to enlarge. In addition, the enlarged uterus causes increased pressure on the veins. Varicose veins due to pregnancy may improve within several months after delivery, but never go away completely. Other predisposing factors include: occupations involving prolonged sitting or standing, obesity, trauma, and hormonal factors.