Because they didn’t have their body subjected to those last, most difficult months of pregnancy. My own surprise (but very wanted!) pregnancy was easy the firs

Because they didn’t have their body subjected to those last, most difficult months of pregnancy. My own surprise (but very wanted!) pregnancy was easy the first few months - I even skied up through week 16, but around week 30 I started developing back pain and intermittent numbness in some of my toes that is still around, after nearly two years and lots of physical therapy. And I was always considered to be having a “healthy” pregnancy, and was pretty healthy beforehand. Only unusual thing is that I was 40. For a more extreme but still realistic hypothetical, I have a severely physically disabled friend who weighs about 60 lbs and requires regular mechanical breathing assistance. She is, as far as she knows, just as capable of getting pregnant as I am. She also knows that she would not survive more than a few weeks of pregnancy. There are many people capable of becoming pregnant who live somewhere between these two extremes.