One of the most important dates in a woman's life can be the "due date", that incredible day when her baby is expected to arrive.

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.

One of the most common reasons for this due date likelihood of being incorrect is that many women do not have regular menstrual cycles. The pregnancy due date is calculated using an average menstrual cycle of every 28 days and the ovulation date being mid-cycle. For many women this will mean that the estimated due date can be as much as a month off. It is important not to depend too much on the first due date that your health professional gives you. Many pregnancies do not last for precisely 40 weeks. It is quite common that the actual pregnancy due date is longer than this by up to two weeks.

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.

The amount of hCG in a woman's blood will tell her healthcare provider how far her pregnancy has progressed. Therefore, a blood test can be a more accurate way to gauge the due date than a simple estimation of conception and menstrual cycle. The date determined by a blood test is considered to be the first "true" due date, and you should be able to start planning with this date in mind. It's important to remember that sufficient levels of hCG are not present in the blood early in pregnancy, so this test is not available during the initial stages.

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.

Once you receive a due date based on your ultrasound scan or blood tests, you can expect future estimations to vary by just a day or two. Every mother and baby is different, of course, so your child may arrive a day or two before or after this estimated date. Your baby's birth will probably not take place on the due date, so don't make any hard and fast plans that require a specific 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.

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Writer Rosamond Gaven writes for several well-known web sites, on health plan and health and wellness themes.