Now that the Duchess of Cambridge has been admitted to St. Mary's Hospital in "early labor," the next logical question becomes: How long will this take?

It's a practice that has existed probably since the beginning of time, trying to predict when a baby will be born after a woman has gone into labor. In modern times, there are delivery "pools" (not the water type) in which people bet or guess on all aspects of the baby's arrival: date, hour, weight, length and gender.

But in the medical field of obstetrics, there are actual ways that we can predict how long it will take a woman to deliver. The method we use provides an estimate of time, in hours, for labor for two scenarios: a first baby's delivery and subsequent deliveries. Called the Friedman Labor Curve, it is a rough estimate of a woman's time course during labor.

It reveals the amount of time, in hours, that an average woman would take to progress through 10 centimeters of cervical dilation, and then actually to push out and deliver the baby. But there are real issues with the Friedman curve that have brought its validity into modern-day question.

For one thing, when this labor curve originated, not as many women used epidurals for pain relief. There are experts who say that this is a major factor in making the data of the curve somewhat outdated. Nevertheless, there are averages, and we do use these parameters in modern-day obstetrics as a rough guide with which to assess the progress of a woman's labor course.

For a first baby, 50 percent of women progress at a rate of 1-1.2 centimeters of cervical dilation per hour. For subsequent babies, 50 percent of women progress at a rate of 1.5 cm of cervical dilation per hour. Remember, the curve is based on averages and the curve relates to what's known as the "active stage" of labor, which is the time period between when the cervix is roughly 6 centimeters m and when the cervix is fully dilated at 10 centimeters.

The next stage of labor is known as the second-stage of labor and refers to the period of time between when the cervix is fully dilated, and when the baby is born. For a woman having her first baby, without an epidural, we usually expect up to two hours for this stage. With an epidural, we allow up to three hours. But, again, this will vary woman to woman.

Overall, labor and delivery comes down to the 4 Ps: power, passenger, pelvis and psyche. The power is how strong the uterus contracts to cause the cervix to dilate. Passenger refers to the fetus and its size and position. The size and type of pelvic structure is also critical as is the mother's mind-set (psyche). Using a combination of physical examination, the obstetrician or midwife can assess these four variables and, thus, support the woman in labor.

The exciting thing about labor and delivery is that there are averages and statistics but ultimately what happens to each woman can be unexpected and unique. Obstetrics is as much an art as it is a science and any obstetrician or midwife will agree that where these two factors meet becomes the essence of labor and delivery.

That is what becomes exciting.