Early in the book it is mentioned that Mr. Craven's wife died ten years ago because she was reading in a tree and the branch she was sitting on broke. Later, it is said that Mr. Craven couldn't bear to look at baby Colin (who in the book's present is ten years old) because he reminded him of Lilias. The book never overtly connects the dots any more than that, and I must have read it a million times before I realized that Lilias not only died in childbirth brought on by the fall... but the fall itself was due to the fact that the branch couldn't support her weight in the later stages of pregnancy. -Ella V
Martha also mentions when relating the story to Mary that Lilias was a small, petite woman, so it’s possible that she thought the extra weight wouldn’t make that great of a difference. - Baby Moondancer
It's also entirely possible that the reason everyone's so concerned about Colin's health is because of the fear of damage from the fall.