Abortion and a woman's mental health?

A longer discussion of the study can be found here.

The University of Otago study found women who had an abortion faced a 30 percent increase in the risk of developing common mental health problems such as depression and anxiety. . . .

the findings have implications for the legal status of abortion in New Zealand where over 90 percent of terminations were authorised on the grounds that proceeding with the pregnancy will pose a serious threat to the woman's mental health. . . .

I haven't read the study, but my concern is that the same factors that caused the woman to have an abortion might be related to whether she has mental health problems. It seems like a difficult problem to disentangle.