I was pregnant with my second child when I began having constipation. It turned out to be my first symptom of rectal cancer, but I didn’t know that at the time.

Each night, I’d wake up with an urgent need to go to the restroom. But then I’d sit on the toilet, unable to empty my bowels all the way. It was annoying and interfered with my sleep. My obstetrician explained that when a woman is pregnant, her expanding uterus takes up...