How your due date is calculated is often confusing and different from what you might expect. Pregnancy is 40 weeks, or 280 days, long when measuring from the first day of the last menstrual period. But conception occurs usually two weeks after the first day of the last menstrual period. This means that at the time of conception, a woman is already two weeks pregnant when measured this way. This assumes that her menstrual cycles are 28 days apart.

There are plenty of pregnancy due date apps and calculators that will tell you your estimated due date (EDD) based upon the first day of your last menstrual period (LMP). There’s even one on the front page of Calculating your EDD from your LMP assumes a few things:

  • That you remember the first day of your last menstrual period correctly;
  • That you weren’t on birth control or something that would alter your cycles at the time of conception;
  • That you have regular, 28-day cycles.

If your cycles are shorter or longer, your due date must be adjusted. If you don’t remember when your period was or you were on birth control at the time, you will probably have to be dated by an ultrasound. The earlier the ultrasound is performed, the more accurate it is for estimating your due date. Your doctor or midwife will help you figure out an exact due date at your first or second visit.

Many times, women don’t remember the exact day of their last menstrual period, or they may not have menstrual cycles that are exactly 28 days long, or they might have been on birth control at the time of conception. Sometimes women ovulate a little bit later than usual, or the bleeding they thought was their last menstrual period was actually related to the pregnancy (e.g., implantation bleeding). All of these factors mean that dating based on the last menstrual period is wrong about 40% of the time. Your doctor or midwife will determine your due date based on your last menstrual period and compare that to a due date determined by the earliest ultrasound, and in some cases, your due date will need to change based on the ultrasound.

As we said, the earliest ultrasound that you have is the best one to determine your due date. Sometimes, patients are confused because subsequent ultrasounds will show measurements that have a slightly different due date and they will wonder if their due date should be changed again based on these later ultrasounds. The answer is no. The later ultrasounds reflect a pregnancy that is just a little bigger or smaller than the average for that gestational age or vary just because of the margin of error of the scan.

Make sure you clarify at one of your first prenatal visits what your final estimated due date is; then, don’t get too fixed on that day. Only 2% of women deliver on their due date. Maybe we should’ve called it a due month!