Anonymous wrote:OP here.
I think my OB explained it to me this way... IUDs are supposed to keep sperm from meeting egg (of course), but in the very rare cases that conception happens, the IUD almost certainly prevents the cells from attaching to the uterus. so you're basically setting up those rare cases of conception to fail.
that's the thinking behind the no-IUD policy, anyway.
That is correct.
IUDs do not prevent ovulation. They make the uterine environment less hospitable to sperm. However, if conception does occur, (which happens an indeterminate number of times), the IUD is supposed to "scrape" the implanting embryo out, or make the uterine lining too irritated for implantation to occur. Of course, sometimes the embryo does implant and continue to grow, but IUDs can continue to threaten the pregnancy as it progresses.
Every birth control besides barrier devices has an anti-implantation element; in other words, they do not just prevent conception, they can also disrupt a new pregnancy. Church teaching is that artificial contraception and sterilization are wrong, and
also that life, once begun, must be protected from the moment of conception. Therefore, Catholic institutions cannot participate in such actions without materially cooperating with grave sin.