Calculating Your Due Date
There are many methods for calculating your due date when pregnant. One of these methods is the Last Menstrual Period Method, which is often abbreviated to LMP. With this method of calculation, in order to find your due date, you simply count forward 280 days from the first day of the last menstrual period you had. This method takes into account the average gestation period, which is 266 days, plus an additional two weeks, to account for the time it took for you to ovulate and be fertilized.
The Last Menstrual Period Method looks at what is known as gestational age, which takes into account the time it took for the egg to be released from the ovaries and then fertilized. The age of the fetus is not the same as the gestational age. The age of the fetus is calculated to be two weeks younger than the gestational age.
This is an easy method to use, provided you remember the starting date you need to use, but according to pregnancy-period.com http://www.pregnancy-period.com/pregnancy-calculator.html, it may be somewhat inaccurate as well, because not all women take two weeks to ovulate after the first day of their period, and it can be hard to tell on exactly which date fertilization took place.
The Accuracy of the Last Menstrual Period Method
This method is best used if you have a menstrual cycle that is regular and reliably runs in 28-day cycles, with ovulation regularly occurring right in the middle of that cycle, on the fourteenth day. However, many women do not have cycles that are that regular. This makes using the Last Menstrual Period Method problematic for them. According to pregnancychildbirth.suite101.com http://pregnancychildbirth.suite101.com/article.cfm/last_menstrual_period_dating, women who have shorter menstrual cycles may calculate their due date to be farther off than it is, while women with longer menstrual cycles may calculate it to be nearer than it is.
There are other factors that can get in the way of the accuracy of the Last Menstrual Period Method, including bleeding in between periods, a mild form of which some women do experience, sometimes as a result of ovulation. It also may be difficult to use this method if the woman has been on birth control pills or is breastfeeding, because hormone levels are altered in these two situations, compromising the woman's ability to fully understand what is happening with her own body at any particular time. It has even been found among some studies detailed on medscape.com http://www.medscape.com/viewarticle/703501_2 that even among women who have normal cycles that run for 28 days, ovulation does not always occur in the middle of this cycle, which would make this method an inaccurate way to determine the due date for most women. A preferable method of calculating due date would be using ultrasound, which does not depend on menstrual regularity to make accurate predictions.