Pregnancy
Apart from causing hormonal fluctuations, pregnancy increases the amount of blood flowing in the body. This in turn, can cause dilation of the veins. The enlarged uterus can also compress the large vein on the right side of the body, which can increase pressure inside the leg veins, and cause them to dilate. Sometimes, the valves inside the veins may fail to function properly due to this dilation. As a result, blood can pool inside the veins and cause the development of spider and varicose veins.