You're officially in active labor when your cervix is four centimeters dilated. Some women can stay just shy of this stage of dilatation for days or a week or two before they experience consistently regular, hard contractions. So we will arbitrarily say your labor has begun when your contractions become regular and increasingly intense, and you are likely to see your baby within a day.
We do not find the terms "true" and "false" labor helpful, nor accurate, since there is no such thing as a "false" labor contraction. As discussed, all those prelabor Braxton-Hicks contractions you've had for weeks and months have been toning the uterus, adjusting baby's position, and effacing your cervix, all preparing for the day you're going to labor a baby out. Instead, we find it helpful to divide contractions into preparing-the-passage-for-baby contractions (prelabor contractions) and delivering-baby contractions (labor contractions). Many women, especially first-timers, can't pinpoint the exact moment labor contractions begin. Labor contractions can seem like prelabor ones at first. After the fact, of course, mothers can look back and say, "Oh yes, that was when they started." Once active labor is well underway, you'll no longer doubt that this will end except with the delivery of your baby. Here's how to tell the difference.
Prelabor contractions (also called "false" contractions):
Labor contractions (also called "real" or "true" contractions):