In most cases no, unless you are unsure how long you have been pregnant or you are at an increased risk of ectopic pregnancy.
It is important to know approximately how advanced your pregnancy is if you are opting for an abortion, because different gestational ages require different abortion methods. For example, abortion pills can only be safely used at home in the first 12 weeks of pregnancy. Women who have regular menstrual cycles of around 28 days can get a fairly accurate estimate of how long they have been pregnant by counting the days and weeks from the first day of their last menstrual period (LMP).
Women with irregular cycles, or women who become pregnant on hormone contraceptives, cannot rely on this method to estimate how long they have been pregnant. These women should consider getting an ultrasound, because an ultrasound can accurately determine how advanced the pregnancy is.
The other situation when an ultrasound could be useful before an abortion is to rule out ectopic pregnancy in women who are at increased risk of one. In an ectopic pregnancy, the fertilised egg implants outside the uterine cavity, usually in the fallopian tubes. Some situations, such as becoming pregnant with an IUD in place or having had previous ectopic pregnancies, increase the risk of ectopic pregnancy. Women with an increased risk of ectopic pregnancy should have an ultrasound to rule out an ectopic pregnancy before having an abortion. This is because the abortion pills will not terminate an ectopic pregnancy and you would be at risk of the ectopic pregnancy rupturing.