Why was Mrs. Copperfield sad in David Copperfield?

We learn early in the novel that Mrs. Clara Copperfield is

very timid and sad, and very doubtful of ever coming alive out of the trial that was before her . . . .

She is sad because her husband has died, leaving her all alone, but also because she is pregnant with her first child, David, and fears the labor and delivery will kill her. She is very young, not yet twenty. When her husband's aunt, Betsey Trotter, arrives at this point, she exclaims that Clara is a "very Baby!"
Clara Copperfield's innocence and youth are emphasized, as they will cause her more sadness as the novel goes on. She has a kind-hearted, generous spirit and will be lured into a marriage with the hard-hearted and cruel Mr. Murdstone. She will be unable to resist or fight back when he and his rigid, equally cruel sister take over the household and tyrannize her and David.