Congratulations, you are pregnant. One of the most frequent questions you will be asked over the next nine months will be, "What is your due date?" For some women, especially those who were actively trying to conceive, this may be an easy question to answer, but other women may have to look to their physicians and health care team to provide them with an accurate due date.
Expected Date of Confinement
You may hear or read this term, also abbreviated EDC, in online medical journals, in your physician's office or in your medical records. Expected date of confinement simply refers to your due date. Your due date is the best approximation, based on medical statistics, of when you will deliver your baby. A lot of work and love goes into preparing for this special day. Only 5 percent of newborns, however, actually arrive on their EDC.
Naegele's Rule
Naegele's Rule is a simple formula for the calculation of a woman's due date based on her last known period, or LMP. There are many online calculators that can perform this task for you, but the simple equation looks like this:
Due Date = LMP + 9 months + 7 days
So, if your last period began on Jan. 5, to calculate your due date, add nine months and seven days for a due date of Oct. 12.
Ultrasound Testing
If you do not know the first day of your last period, or if your menstrual cycles are irregular, Naegele's Rule is not for you. The next best tool for determining your due date is ultrasound examination. David L. Cutzler, a certified nurse-midwife, explains on the website All About Due Dates that the reliability behind ultrasound due date predictions comes from the underlying statistical data comparing fetal growth at various stages of gestation. In other words, ultrasound makes it possible to measure the size of your fetus, and compare its size statistically to the growth of other fetuses with known due dates to give you a predicted due date. This exam may be repeated at a later date, and your results may be compared to confirm or revise your EDC.
Accuracy of Ultrasound Dating
Due to normal differences in growth rates, ultrasound dating is not more reliable than dating a pregnancy based on your last menstrual period. Do not be alarmed if the due date calculated by Naegele's Rule varies slightly from the date your ultrasound examiner determines.
Blood Tests
Qualitative hCG blood tests measure the actual amount of the pregnancy hormone hCG present in the blood stream. Levels of hCG rise at predictable intervals, doubling every 48 to 72 hours. This can be a good means of determining how your pregnancy is progressing over time, but it is not a reliable means of determining the age of the pregnancy. As with individual fetal growth, hormone levels vary greatly, and slightly low or high readings may mean little to your developing pregnancy. Contact your physician if you have any concerns.
