Is it true that IUD's like Mirena cause abortion when sometimes a sperm does fertilize an egg?

Mirena has multiple mechanisms of action. The hormone in the IUD may sometimes stop the release of an egg from the ovary, so in these cases the egg is present to be fertilized. However, this is not the way it usually works as ovulation still occurs in most women.

It is also thought that the Mirena system may prevent sperm from reaching or fertilizing an egg. However, some sperm may reach the egg anyway, resulting in fertilization.

When fertilization does occur, Mirena is thought to act as birth control by preventing the embryo from implanting in the uterus. If this is what you mean when you say "abortion," then YES, this does happen. (However, keep in mind that is it technically not called an abortion unless the demise of the embryo happens after implantation.)

It is not known which of these mechanisms is most important for preventing pregnancy and most likely all of them occur. Some women choose not to use the IUD because of this concern.

Best of luck,
Dr. Williams