Abortion rates also vary depending on the stage of pregnancy and the method practiced. In 2003, the Centers for Disease Control and Prevention (CDC) reported that 26% of reported legal induced abortions in the United States were known to have been obtained at less than 6 weeks gestation, 18% at 7 weeks, 15% at 8 weeks, 18% at 9 through 10 weeks, 9.7% at 11 through 12 weeks, 6.2% at 13 through 15 weeks, 4.1% at 16 through 20 weeks and 1.4% at more than 21 weeks. 90.9% of these were classified as having been done by "curettage" (Suction-aspiration abortion, dilation and curettage, dilation and evacuation), 7.7% by "medical abortion" means (mifepristone), 0.4% by "instillation abortion" (saline or prostaglandin), and 1.0% by "other" (including hysterotomy abortion and hysterectomy). According to the CDC, due to data collection difficulties the data must be viewed as tentative and some fetal deaths reported beyond 20 weeks may be natural deaths erroneously classified as abortions if the removal of the dead fetus is accomplished by the same procedure as an induced abortion.. What are the four different classifications of abortion?