Determining the due date of your baby isn't an exact process. In fact, there are actually a small precentage of babies who are actually born on their due dates. Most obstetrician's offices have due date predictor charts and calculators to help you figure out the approximate date that you can expect your baby. The normal range of birth can fall anywhere from 38 to 42 weeks, however, making the calculation of a due date difficult to hit exactly. Most due date calculators take the first day of a woman's last period and add nine months and one week to determine the due date.