This is a very important matter that is widely misunderstood:
The Church considers a miscarriage to be a physical evil. Since abortion DELIBERATELY causes a miscarriage, it is therefore also a MORAL evil. The Church sees an UNintended miscarriage as only a physical evil since it is not deliberately caused by the couple.
The use of the pill for medical reasons may cause an UNintended miscarriage. Women often have unintended miscarriages—sometimes without even knowing it. It is only miscarriages that are INTENDED that the Church considers immoral. The Church never allows the pill to be used as an abortifacient. But it does allow the use of the pill for medical reasons with the possiblity of producing an unintended miscarriage—without obliging the couple to abstain from sexual relations during that time.
Fr. Vincent Serpa, O.P.