Even your obstetrician is not going to be able to give you a pregnancy due date calendar that's exact. That baby of yours has a mind of her or his own. A pregnancy calendar can help you make a fairly accurate guesstimate of your due date, however.

There are two ways in which you can use the due date calendar for your pregnancy schedule. You can enter the first day of your last menstrual period into the calendar and then calculate the delivery date forward. The results of working this due date calendar give you several important points of your pregnancy schedule. You'll know not only the probable date you conceived, but the end of your first trimester, the date by which you should be starting your birthing classes, the end of your pregnancy's second trimester, and the best estimate of your date of delivery.

Once your doctor is able to estimate your delivery date you can also use this due date information to let the calendar help you estimate your pregnancy conception date.

Once you've used the due date pregnancy calendar to determine where you are in your pregnancy, you'll be able to know what your baby is doing at each point. In your first month your baby develops a circulatory system that works, and her or his spinal cord closes. Motor activity develops the next month, and the baby develops her or his nervous system as well. At the same time the heart's four valves and the circulatory system starts its development. Once you're sure you're ending your first trimester, for instance, you'll know that your baby is able to jerk her or his body and flex her legs and arms. He'll also weigh about the same as a common letter size envelope - less than one ounce.

Once you know you're in your fourth month you'll know as well that your baby is about six inches long and has fingernails and toenails. His or her legs and feet are completely shaped and her hair has stared to grow.

By what your pregnancy due date calendar determines is the end of your second trimester your baby has grown to 12 inches long, has a working inner ear and can grasp with her thumb. One month later the baby's sense of taste has developed, and a month later the lungs are working. During these last two months the baby's weight triples. While the accurate calendar due date pregnancy is 40 weeks in length the common range is anywhere between 38 and 42 weeks after conception.