Once a woman knows she's pregnant, it is important to measure how far along her pregnancy is. This will have an impact on your options, as certain methods of abortion are available only in specific time-frames. It will also help you know how much time you have to make your decision.
Health professionals usually measure a pregnancy in terms of the time that has passed since the first day of the woman's last menstrual period (abbreviated as LMP). If you go to a clinic, they may use an ultrasound machine or a physical exam to establish how far along you are.