Few among us would underestimate the importance of nomenclature, titles, and psuedoscientific calculations in the shaping of our patients' perceptions regarding the due date. A week rarely passes in which a patient does not raise the issue of going past her due date. Our lengthy responses include: “It is only an approximate date,” or “The baby is not really overdue the day after its due date,” or “It is a range of dates.” Patients regard the due date as a definite and specific point in time at which an occurrence or an event is supposed to happen. Our feeble rationalizations rarely satisfy the pregnant woman's anxiety about the health of her child and the desire to have that child safely delivered. In our culture, being overdue is not a desirable quality. This discussion deals with the issue of abandoning the term “the due date” and its associated problematic acronyms, the estimated date of confinement (EDC) and the estimated due date (EDD).
The issue of a pregnancy due date is confounded by the fact that there exists inherent contradictions in the way in which practitioners and patients view pregnancy dating. As clinicians, we employ fractions of the EDD as an estimated gestational age (EGA). We use this fraction to measure how far along a pregnancy has progressed. Indeed, this is a critical parameter. We use menstrual cycles, ultrasound size, and obstetric markers such as fundal heights and audible heart tones to assess how far a pregnancy has progressed. When we know how far a pregnancy should be, we can assess it using Goldilock's rule to interpret whether the pregnancy is too big, too small, or just right. Additionally, knowing the EGA, we can make more reliable management decisions. For example, we know when to give Rh immune globulin, when to test for glucose intolerance, or when to start antenatal testing. We use dating to see how far a patient has progressed. In contrast to our use and view of dating, patients use the due date as a marker of how far a woman needs to progress to be delivered. The due date given by the physician or midwife is a very real and specific point in time at which to expect delivery. Patients do not use pregnancy dating to measure progress, but they use it to set expectations. If the delivery by the EDD is not met (and commonly it is exceeded), then anxiety sets in. In reality, we do not know when a baby is due to deliver, and we do not know any health provider who truly believes that a due date is predictive of the day a baby will arrive.
There are several arguments for the abandonment of the term “the due date.” The concept of the due date was never empirically established. A recent report noted that Nagele's rule of 280 days, or 40 weeks, was derived by a 17th century botanist, Booerhave, who read in the Bible that pregnancy should last 10 lunar months.1 Several large studies have found that predicting a delivery date may be more accurately accomplished by ultrasound dating in the second trimester.1–3 One report noted that the most common delivery date would be 283 days from the last menstrual period.3 Using predictions based on menses has always been filled with flaws. Even if delivery would occur 38 weeks after conception, how often does conception occur exactly 14 days to the minute after the first drop of menstrual blood appeared? The due date is an average delivery date. Only 5% of mothers will actually deliver the baby on their due date.
Additionally, some pregnancies should be delivered on dates other than a 40-week due date. For example, strong consideration should be given to delivering a mother with twins at 38 weeks. Other women with scheduled cesarean deliveries have delivery dates around 39 weeks. Women with severe hypertension and insulin-dependent diabetes mellitus may have due dates earlier than 40 weeks as well. Thus, due dates commonly are calculated and then altered, further increasing patient frustration and dissatisfaction.
Many deliveries are scheduled inductions. Our patients lead increasingly complicated lives and they desire help in planning when to have the delivery. This is one of the main reasons that have led to the increase in elective inductions. Grandparents who are coming from out of town need to schedule ahead of time to get the lowest airplane fares. When spouses and partners must schedule time off work, knowing a reliable due date is very helpful. Another factor, which has led to the increase in social inductions, is the increase in size of obstetric groups. There is a desire to have one's own physician perform the delivery. The bond between accoucher and patient has existed for centuries. Most women wish to have the person in whom she has put her trust help with the arrival of her child. How can a physician know the call schedule 9 months in advance? Additionally, with the increasing nursing shortage becoming a concern for labor and delivery, it makes sense to schedule inductions when more nurses can be available, such as between Monday and Friday. Thus, our current reliance on a menstrually calculated, poorly predictive, 280-day due date seems ineffective, outdated, and counterproductive. We beg the question, why predict a single delivery date 10 months before the actual date?
We believe that a gestational age assessment, whether clinically, biochemically, or sonographically determined, is a necessary and helpful part of pregnancy management. We should not abandon the EGA. However, the due date should be eliminated! At most, the EDC or EDD is a nuisance, and at worst it is a total pain. A due date should not be a single day established early in pregnancy. It has always been considered to be a range of dates. We propose that at 32 weeks' gestation, a patient should be given an assigned week of delivery (AWD). An AWD will be helpful for all the parties concerned. The criteria for establishing an AWD will be individualized for each patient and each clinical situation. For example, if the patient has a normal pregnancy, we may give her an AWD between 39 1/2 and 40 1/2 weeks. A typical response would be: “Ms. Jones, your AWD will be the week of October 17.” If a patient has twins, she could have an AWD that would correspond to 38 weeks' gestation. A primiparous woman may have an AWD between 40 and 41 weeks. Statistics in nurseries and in medical records can document the EGA of delivery, just as they do now. However, the patient and her family can make their plans based on her AWD. Using 32 weeks' gestation allows enough time for work schedule adjustments and good airplane fares but does not overly inhibit appropriate clinical estimation. Additionally, night call schedules are set at least 8 weeks in advance.
Ultrasound machines should never print the EDC. No machine has the right to give the EDC. Only a human being should assign a time for delivery. The EDC should be abandoned, and an AWD should be set by the health care professional based on the needs of the patient, the needs of her family, and the needs of the medical practice. This will eliminate the obnoxious problem of a patient coming in and announcing that she has six due dates, each given by a different ultrasound examination. An AWD can always be adjusted as necessary because it is an assigned week, allowing for greater flexibility. It is not a due date that is based on a presumed visualization of vaginal bleeding.
Every good cook knows that the time a cake needs to stay in the oven varies by the type of ingredients, large eggs versus small eggs, altitude of the oven, and the type of cake that is being prepared. To assume that every cake needs to be 350C for 40 minutes is only a sign of a lousy cook. We would not want to eat that dessert.
It may initially seem onerous to assign an AWD instead of using the EDC, but in reality, we already reassign people's dates all the time and we spend more time attempting to correct patients' expectations. An AWD allows us to be more realistic and our patients to be more trusting. Lastly, we would like to offer seven reasons why a 7-day period of time, a week, is preferable for an AWD:
- Keeping the idea of weeks, which is traditional, reinforces our historic legitimacy.
- There have been seven great advances in obstetrics.
- Many women know the week of their last menstrual period, but do not know the exact day.
- There are seven branches of medicine.
- Kirk-Edward's rule. There must be seven legitimate reasons to induce labor. (Personal communication. M. Edwards.)
- One should strive to avoid the seventh deadly sin: the woman who presents in labor with multiple due dates.
- Seven is a lucky number.
Questions may arise over time as to the reliability of a practitioner's AWD. We believe that the assigned week should correspond to 95% of all births. This statistic is derived from cesarean birth rates, induction rates, rates of medical complications of pregnancy, and the strong desire in patients and their families to deliver on time. However, local rates may vary. We do not see a need for the creation of seminars for accreditation in establishing AWDs because current medical training already teaches the assessment of patients as part of good obstetric practice.
In summary, the inherent contradiction of a patient's desire to know when she is delivering versus the physician's need to know how far advanced a pregnancy is leads us to believe that we should separate the EDC from the EGA. Additionally, because no one can predict a day of delivery, each patient's delivery should be assessed and assigned through our skill and training, rather than arbitrarily calculated. We believe that offering patients an adjusted week of delivery at approximately 32 weeks will alleviate the frustrating and time-consuming problem of the EDC.
1. Baskett TF, Nagele F. Naegele's rule: A reappraisal. Br J Obstet Gynaecol 2000;107:1433–5.
2. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: Ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol 1996;174:278–81.
3. Bergsjo P, Denman PW, Hoffman HJ, Meirik O. Duration of human singleton pregnancy—A population based study. Acta Obstet Gynecol 1990;69:197–207.