A pregnancy is considered to be full term at forty weeks. Therefore, your estimated due date is said to be forty weeks after the date of conception. Of course, this system is an estimation, and may result in a somewhat inaccurate due date.

Many women have irregular menstrual cycles, which can often lead to an incorrect due date estimation. Generally, a baby's due date is calculated with a 28-day menstrual cycle, and it's assumed that ovulation has occurred midway through the cycle. This formula just doesn't work for all women. In fact, the estimated due date can be off by as much as a month, so don't rely too heavily on this initial date. Your pregnancy may be longer or shorter than forty weeks, and it's not uncommon for some mothers to have a true due date up to two weeks longer than the initial estimate.

A rough pregnancy due date is enough for most women to plan when they are going to go on maternity leave. Even if the due date isn't entirely accurate it can be used as a good indicator of when the women can decide to stop working. As the pregnancy progresses the pregnancy due date can be estimated much more accurately by either a blood test or an ultrasound scan.

Blood tests are able to pinpoint a due date more accurately. This simple test will measure the amount of hCG that a woman has in her blood, which in turn indicates how far the pregnancy has progressed. The results of these tests can be considered to be more accurate than those gained by estimations of ovulation and menstrual cycle. Most women can safely use the date from a blood test as a truer indication of when baby will arrive. Remember, hCG levels are not high enough in early pregnancy, so this test can only be done during the later weeks.

An ultrasound scan is the most reliable way of determining a pregnancy due date. This is because babies develop at a relatively uniform rate in the second trimester. The ultrasound technician will take pictures of the unborn baby and take a number of measurements needed to estimate the due date. The pregnancy due date is calculated by the length of the fetus and the circumference of its head, as well as other points of reference.

As your pregnancy progresses, your due date should remain fairly consistent, give or take a day or two. Remember, any date that you receive is to be used as a guideline only. Most babies are born at least a day or two before or after the due date.

There is no way to guarantee the exact date of your baby's arrival. After all, life's most precious moments are often left to chance.