Is abortion okay, if the mother's life is at risk, according to Catholicism? Is it okay to abort a baby when the continuation of the pregnancy may result in the death of his or her mother, according to Catholicism?
In Catholic morality, abortion is always wrong.  Another principle is that intention is primary: it's about saving lives.  When in saving the mother's life and reasonable care has been done for the unborn baby but the baby still does not survive, it's not called abortion as it is the unintended side effect.
The article A Dangerous Pregnancy and Abortion goes over the decision making process when there is a dangerous pregnancy that can result in the death of the mother, the unborn baby, or both.
I freely quote from the article above:

Once pregnancy takes place, there are 2 essential facts: mother and child.  Everything must be done to keep both healthy.  Neither is more important than the other, although the mother already has other children dependent on her.
Early delivery option can take place even it poses a certain risk to the child, as long as reasonable care is taken.
Example when both are in jeopardy: In the case of tubal pregnancy, the most common of ectopic pregnancy, if the medically necessary intervention to save the mother's life is to remove the portion of the Fallopian tube that is either ruptured or soon to be ruptured, it is permissible even though this will also remove the human embryo.  But this is not called an abortion since this is not the intention, using &quot;the principle of double effect&quot; (meaning one thing is intended and another is not).  See more discussion on the &quot;principle of double effect&quot; here.

Let us not forget of the reverse scenario where the mother remains free to make a morally licit decision to risk her own health to save her baby's life, as Ken Graham's answer shares the story of St. Gianna Beretta Molla who powerfully showed how an unborn baby's life is as precious as the mother's life, and how her testimony of sacrificial love has inspired many in the world today as an excellent imitation of Christ's sacrifice for us.