RICHMOND, Va. (AP) – Virginia’s rate of babies born prematurely has reached its highest point in seven years, yet researchers say the reason remains a mystery.

According to the March of Dimes’ annual premature-birth report card, Virginia’s 2016 rate was 9.6 percent, nearly half a percentage point higher than the previous year. The Richmond Times-Dispatch reports Virginia’s grade dropped from a B to a C on the report card.

Nationally, the rate increased from 9.6 percent in 2015 to 9.8 percent, amounting to an additional 8,000 premature births. Among black women, the preterm birthrate was 49 percent higher than other women.

Premature babies are born before 37 weeks. Being born prematurely can lead to several complications for children, including infections and jaundice. They can also have health problems that can affect them for their entire lives.