Why are pregnant women prone to heartburn, indigestion, constipation and haemorrhoids?

Fetus/uterus growth: Heartburn and indigestion are caused by the increasing abdominal pressure, due to the enlarging uterus. Hemorrhoids are caused by increased pelvic venous pressure - same cause. Not all pregnant women become constipated, but the fetal and uterine enlargement surely can lead to GI distress. I suggest a good diet and good hydration. Good luck.