First Day of Last Menstrual Period
LMP stands for the first day of your last menstrual period. This is used to calculate how far along you are in your pregnancy. Your LMP is considered day one of your pregnancy, even though you may not have actually conceived yet. The average length of a human pregnancy is 280 days, or 40 weeks. So, if your LMP was on January 1st, you would be due around October 8th.
LMP can also be used as a dating method for other purposes, such as calculating how far along you are in your menstrual cycle. If you have a 28-day cycle, for example, and today is Day 14, then your LMP was 14 days ago. This method is not always accurate, however, as many women have irregular cycles.