Here are 4 reasons. 1. The developing fetus causes an increase in pressure on the abdominal and pelvic veins, which in turn is transmitted to the hemorrhoids. 2. Constipation is common during pregnancy, which aggravates hemorrhoids. 3. Hemorrhoids often have estrogen receptors, and the increased estrogen levels during pregnancy causes the tissue to enlarge and become easily inflamed. 4. Straining during delivery.
Vascular congestion. As the pregnancy grows there is increased resistance to flow for blood vessels in the lower extremities and pelvis. This can lead to vascular congestion not only in veins of the legs, but also in the venous plexus of the anus. Congestion in this venous plexus is manifested clinically as hemorrhoids.