Even if the regulation went away, most insurance plans would continue to cover most forms of birth control. Ms. Salganicoff and several colleagues published a report on the state of contraceptive coverage last year. They found that, before Obamacare, 28 states required health plans to cover contraception, and 85 percent of employer plans had it as a covered benefit. The Equal Employment Opportunity Commission ruled in 2000 that any health plan that covered preventive health services also needed to cover birth control. That is the world we would return to if the mandate was rescinded.

There’s a financial incentive for covering birth control, and insurance companies know it. Unlike most preventive health services that health insurance pays for, birth control tends to have a quick and measurable return on investment. Women without reliable birth control access are more likely to become pregnant, and the costs of prenatal care and childbirth are substantial. Even without a legal requirement, most insurers used to cover birth control, and they have a business case to do so.

Without the Obamacare rule in place, coverage could get patchier, and more women might have to pay for a portion of their care or face more limited choices of covered birth control methods. IUDs are relatively expensive devices, and often require a few office visits for counseling and insertion. Insurers tended to charge women more to get them than they did for comparatively less expensive birth control pills. Even some pills could get quite expensive. Co-payments for birth control, before Obamacare, varied widely. Generic pills tended to be inexpensive, but some women were paying $50 a month for the pill of their choice.

A bigger threat to contraceptive access, of course, is the repeal of Obamacare itself. Whether and how the health law would be erased remain unclear. Mr. Trump told The Wall Street Journal on Friday that he hoped to amend the health law rather than eliminate it entirely. A repeal bill passed by Congress last year, and vetoed by President Obama, would leave about 22 million people without coverage, many of them women who are using their health insurance to pay for birth control.

Even an Obamacare repeal would take time to pass Congress, and would most likely take years to be implemented. If you want an IUD now, get one. But they won’t go away any time soon.

“It’s a real threat,” said Gretchen Borchelt, vice president for reproductive rights and health at the National Women’s Law Center. “But let’s not panic.”