Despite the United Nations Millennium Development Goal of a 75 percent reduction in maternal deaths by 2015, the estimated maternal mortality rate for 48 U.S. states and the District of Columbia actually increased by 26.6 percent from 2000 to 2014, according to a new study co-authored by Boston University School of Public Health (BUSPH) researchers.

The study, published online in the journal Obstetrics & Gynecology, found that the estimated maternal mortality rate in the U.S., excluding California and Texas, was 23.8 per 100,000 live births in 2014- up from 18.8 in 2000.

The study found that earlier estimates significantly underreported maternal deaths, largely because of delays in some states' adoption of a 'pregnancy question' on standard death certificates. Because of those delays and resulting discrepancies, the U.S. has not published an official maternal mortality rate since 2007, the researchers said.

"The current maternal mortality rate places the United States far behind other industrialized nations," said study co-author Eugene Declercq, professor of community health sciences at BUSPH. "There is a need to redouble efforts to prevent maternal deaths and improve maternity care for the four million U.S. women giving birth each year."

Declercq and colleagues noted that the World Health Organization has reported that 157 of 183 countries have shown decreases in their maternal mortality rates since 2000. The current estimated U.S. rate is comparable to that of Iran and Ukraine, they said. And among 31 industrialized countries, only Mexico has a poorer rate.

For the study, the research team analyzed detailed mortality data available from the National Center for Health Statistics and the Centers for Disease Control and Prevention. Maternal mortality is defined as the death of a woman while pregnant, or within 42 days of termination of a pregnancy, from any cause related to or aggravated by the pregnancy or its management.

In 2003, a question was added to the standard death certificate to ascertain every female decedent's pregnancy status, even if the woman was not pregnant at the time of death. But only four states revised their death certificates in that year, while adoption by other states was staggered. Because of those delays, there were discrepancies in how maternal mortality was reported.

The research team developed a "correction factor" to adjust the unrevised data so that it was comparable with the revised data. The adjusted numbers of deaths were then used to compute maternal mortality rates for groups of states. California and Texas were analyzed separately because they had trends that were markedly different from other states: California showed a marked decline in maternal mortality from 2003 to 2014, while Texas had a doubling of its reported mortality rate in 2011-2012.

The authors said their calculations indicate that the last official U.S. maternal mortality rate -- 12.7 deaths per 100,000 live births, reported in 2007 -- was significantly underestimated. Their study estimates a rate of 21.3 in 2007, 68 percent higher than the reported average.

"It is an international embarrassment that the United States, since 2007, has not been able to provide a national maternal mortality rate to international data repositories," the research team wrote. "This inability reflects the chronic underfunding over the past two decades of state and national vital statistics systems. Indeed, it was primarily a lack of funds that led to delays (of more than a decade in many states) in the adoption of the 2003 revised birth and death certificates."

The study authors said accurate measures of maternal mortality are key to effective prevention efforts because they can identify at-risk populations and gauge the progress of intervention programs.