Okay, flashback to sixth grade: A pregnancy requires a very delicate ballet to unfold inside the reproductive system. The sperm must find its way to the egg and chassé its way inside it, a process called “fertilization.” (Here’s a very dramatic video of this process, if you’re interested.)

Then, the fertilized egg must attach to the uterus in a process known as “implantation.” The egg grows and grows and becomes a fetus, which hangs out in the uterus until its grand jeté nine months later.

Condoms and birth control pills help with the first part. Condoms keep the sperm from going anywhere. Birth control pills prevent ovulation, or the release of the egg into the fallopian tubes. The so-called “morning-after” pill is just a really hefty dose of the birth control pill, and it interferes with ovulation and fertilization. Both hormonal and copper IUDs do a number of things, including preventing the sperm from reaching the egg and making the uterus less hospitable.

So, does an IUD kill the would-be fetus or simply prevent the egg and sperm from becoming a fetus? It depends on your definition of pregnancy. Most doctors agree that a pregnancy occurs when the fertilized egg attaches to the uterus, but many people, including Hobby Lobby executives, think it happens when the egg becomes fertilized by the sperm. To them, if something interferes with a fertilized egg’s assemblé to the uterus, it’s an abortion.

ACOG representatives told me in an email that copper IUDs mostly work before implantation occurs—copper is toxic to sperm and kills it before it gets to the egg.

But the copper IUD can also be used as emergency contraception if it’s inserted up to five days after unprotected sex (and then simply left in to serve as longer-term birth control). And when used in that way, the copper-laden environment might also prevent the fertilized egg from implanting in the uterus. It might. We just don’t know. The dance of the egg, the IUD, and its molecules changes on a case-by-case and whoopee-by-whoopee basis.

And that’s the part Hobby Lobby takes issue with: They think that “sperm + egg = future baby,” so the fertilized egg’s potential destruction by the IUD is unacceptable.

According to the brief, the Green family, Hobby Lobby’s founders, “each signed a Statement of Faith … obligating them to conduct the businesses according to their religious beliefs, to “honor God with all that has been entrusted” to them, and to “use the Green family assets to create, support, and leverage the efforts of Christian ministries.” And they allow “their faith to guide business decisions.”

This case has any number of ramifications, including that if the Court rules for Hobby Lobby, business owners who have religious beliefs that conflict with blood transfusions or fertility treatments could also hypothetically deny coverage for those procedures, as well.

What’s also interesting here is how the abortion debate often erupts over these micro-moments in human development. Several state legislatures have recently tasked themselves with figuring out exactly when fetuses feel pain, and now a majority-male court must examine a complex contraceptive process that not even the best ob-gyns can predict perfectly. Before, we were arguing about whether life begins at conception, but now we're quibbling over the definition of conception itself.