The abortion rate among American women declined to its lowest level in more than three decades in 2011, according to a new report released Monday that is widely considered the country’s most definitive examination of abortion trends.

The 1.1 million abortions reported in 2011 represented a rate of 16.9 per thousand women of childbearing age, down from 2008, when a similar study estimated that 1.21 million abortions were performed at a rate of 19.4 per thousand women.

Resuming a long-term downward trend that stalled in the middle of the last decade, the 2011 rate was far below the peak, in 1981, of 29.3 per thousand, according to the report from the Guttmacher Institute, a private research group that supports abortion rights.

The decline in abortions from 2008 to 2011 was mirrored by a decline in pregnancy rates. The report did not include a detailed analysis of the reasons for these trends, which pose complicated research issues.