The abortion rate in the United States has fallen to its lowest level since 1973, the year Roe v. Wade was decided by the Supreme Court. A study released Sunday by the Guttmacher Institute reports 16.9 abortions per 1,000 women aged 15 to 44 in 2011. The relatively low rate isn’t because abortion providers are being squeezed but because pregnancy and birth rates have fallen. In addition, “early medication abortions” (the Plan B pill) rose to 23 percent of all nonhospital abortions. The total number of abortion providers declined by 4 percent between 2008 and 2011, and the number of clinics declined by just 1 percent.