Due date calculations are based on clinical estimates that assume a pregnancy lasts 266 days from conception. That said, not all women have a standard 28-day cycle or ovulate on day 14, so these estimates are not always accurate. For this reason, crown-to-rump measurements taken during an ultrasound between eight and twelve weeks provide the greatest accuracy for determining a due date. As the pregnancy progresses into the 2nd and 3rd trimesters, these measurements are much less accurate due to natural growth variations of the fetus. A common misconception about due dates is that once the due date passes, labor should be induced immediately. Many women experience spontaneous labor after their due date, particularly for their first baby. Generally speaking, if fetal monitoring indicates the baby is doing well, and the amount of amniotic fluid is normal, the doctor may allow a woman to remain pregnant until 42 weeks. The actual date of delivery may be affected by several factors, including the number of previous children a woman has had, maternal BMI, and placental health. The position of the baby within the womb and whether or not the mother's hormones are ready for labor are also biological variables that can alter the time frame for labor. Due dates should thus be thought of as a time frame around a given month of potential delivery rather than an exact date.
Due dates are determined by Naegele's Rule, which estimates the date of delivery by taking the first day of the last menstrual period and adding 280 days or 40 weeks to that date. While this is a general guideline for estimating when a baby will be born, the accuracy of the estimate increases significantly when confirmed by a first-trimester ultrasound. Research has found that the most accurate way to date a pregnancy is through an early ultrasound because fetal development at this time is the most consistent across the population. One of the myths about a due date is that it is an "expiration date" for your pregnancy. Many parents think that if they haven't delivered by week 40, there is something wrong, that they are overdue. But a pregnancy is considered term if it is between 37 and 42 weeks. Only 5% of babies will be born on their predicted due date. Many factors will affect the day you actually have your baby, regardless of the estimated due date. If a mother has health problems, like high blood pressure or gestational diabetes, her doctor may recommend that she deliver early. Maternal health and lifestyle choices, along with the mother's past birth experience, can also affect when labor begins because there is a genetic component to the length of gestation.