Calculating the expected due date for a pregnancy just requires simple math.

©iStockphoto.com

Once your pregnancy has been confirmed, what you most want to know is your due date. The average length of a pregnancy is 40 weeks, or 280 days, from the first day of the last normal menstrual period. Calculating the due date, or expected date of delivery, for a pregnancy is quite simple, then, if you know that date. Simply add nine months and seven days to the date, and you've got your pregnancy due date.

Here's an example of how it works: Say the first day of the last normal menstrual period was January 1st. Add seven days to that number, and you get the number 8. Add nine months, and you get October. The expected due date of that pregnancy, then, is October 8. (Some physicians use the term expected date of confinement, or EDC for short, to describe the due date.)

In reality, though, few women give birth on their exact due date. About 80 percent of babies are born within ten days of the due date. That's a fairly large window, beginning ten days before the due date and extending out ten days after the due date. A pregnancy that occurs during this time period--that is, between 38 and 42 weeks--is called full term.

If about 80 percent of expectant mothers have full term births, that leaves about 20 percent who give birth outside those parameters. About eight to ten percent give birth early, between the 20th and 36th week of pregnancy. And about eight percent give birth later than the 42nd week.

A premature, or preterm, delivery usually occurs because the mother goes into labor too early. In most cases there is no clear reason, but abnormally early labor is often associated with the following conditions:

Contrary to popular belief, severe emotional trauma and physical injury, such as from a fall, are uncommon causes of premature labor. If a mother has premature labor in one pregnancy, she has a 25 percent chance of premature labor in the next pregnancy.

The major complication of a preterm delivery is the birth of a baby who is unable to survive, or who has difficulty surviving, outside the mother's body. Even if the baby's organs are all correctly formed, his lungs may not be sufficiently mature to allow him to breathe adequately after birth. Recent advances in the care of premature infants have allowed babies as small as 1 pound to survive and grow up normally. But despite these advances, prematurity remains the leading cause of newborn death.

The cause of postterm pregnancy is unknown. If a woman has had one postterm pregnancy, she has a greater than average chance of this happening again in subsequent pregnancies. In most cases of postterm pregnancy, however, it is believed the mother misstated the exact date of her last menstrual period and the pregnancy is actually not postterm.

Postterm pregnancy poses no health risk to the mother. However, as the placenta ages beyond the 42nd week of pregnancy, its ability to transmit oxygen and nutrients to the fetus may begin to decline. In some cases, this reduction may be severe enough to cause the death of the fetus. If the fetus lives, it commonly has a characteristic postterm appearance: wrinkled, cracking, peeling skin; long nails, abundant hair; and little fat tissue beneath the skin. Postterm babies often pass fecal material called meconium into the amniotic fluid before delivery. If the baby sucks meconium into its lungs at the time of delivery, severe pneumonia may result.

Knowing your due date will help you plan ahead, and it will give you the answer to the number one question that you will be asked during your pregnancy: What's your due date?