Swollen veins… hemorrhoids…can develop from an increase in pressure in the lower rectum. Factors that might cause increased pressure include:  straining during bowel movements, sitting for long periods of time on the toilet and chronic diarrhea or constipation.

 
For obesity, especially in the abdomen and pelvis, may increase pressure on pelvic veins]. Pregnancy increases blood flow to the pelvis and relax supportive tissues while the growing fetus causes increased pressure on blood vessels.

During labor, hemorrhoids may develop because of the intense pressure on the anal area while pushing to deliver the baby]. Medical conditions, for example, long-term heart and liver disease may cause blood to pool in the abdomen and pelvic area, enlarging the veins.
It’s also possible to inherit a tendency to develop hemorrhoids. Hemorrhoids are more likely as you get older because the tissues that support the veins in your rectum and anus can weaken and stretch with aging.

TO BE CONTINUE