40 Weeks From When?
The first question you will be asked when you announce your pregnancy is, “When are you due?” You will mull over this date for many months and plan baby showers, vacations, work schedules and even your wardrobe around it.
An accurate due date is essential in order to have scheduling and interpretation of appropriate testing done during pregnancy, assigning interventions to reduce the risk of preterm birth, ensuring appropriate fetal growth and determining delivery timing if induction of labor becomes necessary.
But you may wonder: How is my due date calculated? Is it reliable? Can it change? How your due date is determined and what to expect follow.
Early Pregnancy Physiology
To determine your due date, you must consider the basics of fertilization and physiology. Many women have a typical 28-day cycle, and the first day of your menstrual cycle is considered day 1. Ovulation is around day 14. Although conception occurs within a few days of ovulation, the due date is actually calculated based on the first day of your last menstrual cycle for standardization among women.
The average human gestational period is 40 weeks, or 280 days from the last menstrual period. So when you have a missed cycle, you are already considered to be approximately four weeks’ pregnant, even though the conception event was two weeks prior.
For women who have irregular or very short or long cycles, this calculation can be inaccurate, and you should inform your healthcare provider of your irregularity. In these cases, or, in the case of an unsure last menstrual cycle start date, the due date may instead be based on an ultrasound done early in pregnancy to establish the size of the growing embryo.
Women who undergo assisted reproductive technologies and have known dates of embryo transfer or intrauterine insemination will be given a due date that takes into account the age of the embryo or technologies used and where the women is in her cycle.
The Role of Ultrasound
There may be times when the size of the baby on the ultrasound is not consistent with the expected due date based on the last menstrual cycle. Ultrasound is considered the most accurate form of dating. Therefore, obstetrical and radiological societies establish guidelines that suggest when a due date should be changed. For example, if the due date based on ultrasound done before 9 weeks’ gestational age differs from the due date by more than 5 days, or by more than 7 days for ultrasound done between 9 and 14 weeks, the date may be changed.
Later in pregnancy, there is a larger margin of error, so the baby’s size by an ultrasound done at 20 weeks should be within 10 days of the menstrual cycle dating. Once a due date is established based on menstrual or ultrasound criteria — ideally early in pregnancy — it should not change, with rare exception.
Timing of Delivery
So will you deliver on your due date? In short, probably not. In fact, only five percent of women deliver on the established due date. Sometimes the baby comes early spontaneously, most commonly after 37 to 39 weeks — but the rate of true preterm delivery (less than 37 weeks) is 12 percent. In addition, there may be reason to induce labor early, such as preeclampsia or other complications. At other times, the baby may come late spontaneously. If nothing happens and no complications arise, your provider will likely recommend that labor be induced one to two weeks after the due date.
Keep this variability in mind when planning work schedules, making travel arrangements for family and other events. The due date is just a guideline. Your baby will not follow your schedule!
Margaret K. Chory, MD, is a general ob/gyn practicing in Pittsburgh, PA.
