Despite pressure from advocates and members of Congress, the Department of Health and Human Services (HHS) will not allow uninsured women to enroll in Obamacare if they become pregnant outside of the three-month window of open enrollment.

Sen. Patty Murray (D-WA) and 36 other senators sent a letter last month to HHS Secretary Sylvia Burwell asking for a special enrollment period for pregnant women, since prenatal care is both expensive and essential for the health of both mothers and children.

The current rules allow a woman to sign up for Affordable Care Act (ACA) coverage outside of the open enrollment period if she is “having a baby,” but that only takes effect after she gives birth. Even a healthy pregnancy can easily cost $10,000-$20,000 out of pocket, and advocates say that such high costs might make women forgo the prenatal care they need to avoid expensive or even fatal complications.

Burwell, in a response sent Friday, said that HHS does not have “the legal authority to establish pregnancy as an exceptional circumstance.”

“It is absolutely critical that all pregnant women have access to health care coverage, so I am disappointed by today’s announcement,” Murray said in a statement. “I will continue looking for ways to get this done so that more women can get covered and get the affordable, high quality care they need for themselves and their young children.”

Burwell’s letter didn’t go into detail about why HHS lacks this legal authority. An HHS spokesperson told Rewire that the department is allowed to create new special enrollment periods for exceptional circumstances, like a natural disaster, that would keep people from signing up in a timely fashion during open enrollment.

HHS concluded that pregnancy doesn’t fit that description.

Asked why HHS can’t just expand the definition of “qualifying life events” to include pregnancy, the spokesperson noted that life events like birth, adoption, and marriage introduce new people into a covered household, and a special enrollment period lets them get coverage.

Burwell’s response said that low- and moderate-income women who become pregnant outside of the ACA open enrollment period can get maternity coverage under Medicaid or the Children’s Health Insurance Program (CHIP).

The ACA requires insurers to cover maternity care to help eliminate discrimination against women in health insurance. But there remain exceptions to that rule, according to a report by Young Invincibles—women might not have maternity coverage if they are insured under certain grandfathered or transitional plans, self-funded student health plans, or a parent’s job-based insurance plan that doesn’t require maternity coverage for dependents.

Medicaid and CHIP eligibility levels also vary considerably state by state, and in many states, $20,000 could still be half the yearly income of someone who makes too much money to qualify for either program.