For every 100,000 births in the US in 2013, the most recent year for which data is available, the Center for Disease Control (CDC) estimates 17.3 mothers died, the highest maternal mortality rate of all industrialized countries—by several times over. But according to another, better estimate, calculated by a team at the Maryland Population Research Center, 23.5 women died per 100,000 live births that year.

The reasons for the discrepancy can be traced back to years of woeful collection of data on maternal health in the US. And while most of the world has drastically reduced maternal mortality in the past three decades, the US is one of just a handful of countries where the problem worsened, and significantly.

America didn’t always fare so poorly in maternal health. Like most other countries, after the end of World War II, the US saw maternal mortality drop—at least until the late 1980s, when the trend reversed. Historic data from the CDC shows the trend clearly: As most countries doubled-down on their efforts to curb maternal mortality, in a drive to meet a United Nations goal to cut maternal mortality by three quarters globally by 2015, the US began moving in the opposite direction. Maternal mortality fell dramatically around the world (30% according to conservative estimates, 45% per the official UN report) between 1990 and 2015, with most progress made after 2000. In the US, the ratio went up nearly 60% over the same period.

Part of the story of why is hidden in the numbers—and the ones about maternal deaths in the US are enough to give epidemiologists a serious headache.

First of all, until the early 1990s, there was no systematic collection of maternal mortality data in the country. Reporting of deaths related to pregnancy or childbirth was limited to the narrow classification listed on death certificates at the time: basically, deaths directly caused by “complications of pregnancy, childbirth, and the puerperium.” The certificate collected no information on whether a woman was pregnant at the time of death or had recently given birth, missing all deaths that occurred during pregnancy for non-obstetric causes, like, say, high blood pressure or depression, as well as those that happen after birth.

It’s estimated that the spotty reporting that resulted missed up to half of all pregnancy-related deaths. Starting in 1991, some states began to introduce a “pregnancy checkbox” to their death certificates. A question answered separately from the cause of death asked:

□ Unknown if pregnant within the past year

Though the effort was uncoordinated, it eventually informed a revision of the US standardized death certificate, which was amended in 2003 to include a question determining whether the woman was pregnant at the time of death or had a recent pregnancy. It took several years for the change to be adopted by all states. West Virginia was the last to incorporate the checkboxes, in 2017.

This means that many more maternal- and pregnancy-related deaths—defined as happening during pregnancy or within 42 days of delivery, or happening up until a year post delivery—have been reported in the past few years, and that’s had an impact on the reporting of national trends.

According to Elliot Main, the medical director of the California Maternal Quality Care Collaborative (CMQCC) and a member of the CDC advisory committee on maternal data, it is likely that the incidence of maternal mortality was higher than the official record showed in the years before the reported numbers began increasing. Perhaps because of that, notes Marian MacDorman, a research professor at the Maryland Institute for Population Control who worked on the mortality estimate, the US missed out on the UN-led effort to curb maternal death, and is now “playing catch-up” with other developed nations.

To complicate things, though the ratio is high, the actual number of cases isn’t. Just a handful of mistakes in reporting pregnancy-related deaths can skew results. As a result, the data collected on death certificates aren’t considered final. State-by-state data are routinely reviewed by Maternal Mortality Review Committees (MMRCs)—review groups in around half of US states, started either as public-health initiatives or by state OB-GYN societies—that look at the records of maternal mortality and assess preventability, as well as whether the deaths are indeed a consequence of the pregnancy, rather than merely coinciding with it.

Unfortunately, the official CDC data are not updated to reflect the findings of the committees, which means the national numbers on record aren’t just likely to be inaccurate—they are known to be wrong. This is why there is such a discrepancy between the estimated ratio, 26.4 in 2015, and the numbers shared by the CDC (in the chart above). It is also one of the reasons why the US has failed to publish an official maternal mortality rate since 2007—”it’s inexcusable,” comments MacDorman.

“It’s not a very clearly reported area,” Eugene Declercq, a professor of community health sciences at Boston University told Quartz. He has collaborated with MacDorman and others to calculate the current estimate (pdf), and founded the site Birth by the Numbers.

In non-scientist terms, data-collection about maternal mortality is a shambles, a symptom of a health care system that has placed insufficient emphasis on maternal health. Quartz in a separate article explored the reasons why American women must risk their lives in greater numbers than their peers in other wealthy countries, simply to have a baby.

In the past few years, however, there has been growing awareness of the tragedy of maternal mortality, and “there is a lot of momentum on the quality of data collection,” says David Goodman, senior scientist for the CDC’s Maternal and Child Health Epidemiology Program. A promising effort, he says, is Review to Action, an initiative to pull together the findings of the MMRCs. So far, he says, review committees exist in 27 states, three more should open within a year, and eight are in the planning. The aim is to soon be able to compare findings.

“[MMRCs] have existed in the US for a century independently from one another,” says Goodman, but “over the past five years [the CDC] has provided coordination, with the goal of coming up with a higher-level review.” The CDC isn’t involved directly in the reviews, but provides technical support, and promotes standardization of data to make it comparable.

Despite the discrepancies in the official rates—and whether or not there has been a steep increase in maternal mortality in the past few years, or instead the number was higher than believed in the past—one thing is for sure, says Main, “Either way we look at them, [the numbers] are high.”