Your Due Date: Science or Black Magic?

Authored by Dr. Cliff Goodman, Jr. on 1/26/2010

Authored by Clifford Goodman

1/26/2010

Categories: New Babies, Health News, MomDocs Online

Categories: New Babies, Health News, MomDocs Online

How do we decide when a baby is due? How accurate are our determinations? Why is the due date important? These are questions we will try to answer in this article. The first and third questions require long, detailed answers. The answer to the second question is “not very.” You will understand why after reading this article.

A Best Guess

As you read this article, please try to think about your due date in terms of a best guess. You go visit your doctor’s office for the first time after you learn you are pregnant. You are happy and excited, and maybe just a little bit scared. You know that your last period started on December 07, and you have decided that you’re probably due on September 14, 2010. How will your doctor confirm or determine your due date?

Definitions

First, let’s make sure that we’re all speaking the same language. Your baby’s due date will usually be referred to (on your medical chart, on insurance and disability forms, etc.) as EDC or EDD. This stands for Estimated Date of Confinement (an old-fashioned name for delivery that holds on only in this sense) or Estimated Date of Delivery. You might also hear or see the term Adjusted or Best or Working EDC, which means the date, based on factors like those discussed in this article, that is the “corrected” date that your doctor thinks is the best guess on when your actual due date is. Our practice uses the term Adjusted EDC for this date. Once this date has been arrived at, it is almost never changed, and it is the one we rely on to make medical decisions.

A Word of Caution

Once this date is established, it is not a pronouncement that the baby will, or even should, come on this date. You must remember a few things about due dates in human pregnancy.

·        Even though, when we look at large groups, the average woman is pregnant for 40 weeks (measured from the first day of her last menstrual period), it is impossible to predict when a particular baby will come.

·        Knowing the date of conception in humans in rarely possible. Although conception usually occurs within hours of when the egg is released from the surface of the ovary, that conception can be from sperm that have waited days, or even weeks, for the egg to appear, or from sperm that make their way there soon after the egg is released. This means that the date of the act of intercourse that results in conception is not necessarily the date of conception. So even in situations where the woman has sex rarely or even only once, the actual conception can occur many days later.

·        Some of our domestic animals, such as cows and horses, have very predictable due dates, but this is not true in humans. Even when the date of conception is known (which is only in certain “test-tube” pregnancies), just one in 25 babies comes on the due date.

·        First pregnancies tend to go late: left alone, about two-thirds of first babies will come on or after the due date.

·        Second (and subsequent) pregnancies fare a little better: left alone, roughly two-thirds will come on or before the due date.

·        If we are reasonably sure about the due date, it is quite rare that you will be allowed to go more than seven to ten days beyond your Adjusted EDC.

Having said all this, it is still important that we do everything we can to have the best and most accurate due date possible. This is particularly important when there is a history of premature labor and/or delivery, a previous Cesarean section, or twins.

The best shot we have of getting an accurate due date is to have a) knowledge about the date of your last menstrual period and how often your periods usually come, especially recently, and b) an early ultrasound (U/S), ideally between six and twelve weeks of pregnancy.

Things Are Often Not as Simple As They Seem (or Why This Article Turned Out So Long)

It is important for us to know, when possible, the date of the first day of your last menstrual period (LMP), how often (interval) your periods usually come, whether they are regular or not, and so on.

We then use the LMP to predict your menstrual EDC with the following formula: Add seven days to the first day or your last period and subtract three months. Thus, in your case, December 07 plus 7 is December 14, and minus three months gives September14, 2010.

This assumes that your periods come every 28 days. If the interval is shorter, the number of days short of 28 is subtracted from the EDC. If the interval is longer, the number of days longer than 28 is added to the EDC. For example, a woman whose periods come every 31 days with an LMP of December 07, 2009 would have an EDC of September 17, 2010. If your cycles are irregular, the menstrual due date becomes very much less important, and we often have to rely completely on the U/S due date.

Prior to about 13 weeks of pregnancy, U/S measures pregnancy to an accuracy of plus or minus 7 days. This means that if the U/S agrees with the menstrual due date within 7 or less days, the menstrual due date becomes your Adjusted (or Best, or Working) EDC). If there is disagreement by 8 or more days, the EDC will be adjusted accordingly. In general, it will not be changed again, even if subsequent U/S measurements disagree (see below to understand why).

From about 13 to 27 weeks of pregnancy, U/S measures pregnancy to an accuracy of plus or minus 14 days. This means that if you don’t have your first U/S until this time, and the U/S agrees with the menstrual due date within 14 or less days, the menstrual due date becomes your Adjusted (or Best, or Working) EDC. If there is disagreement by 15 or more days, the EDC will be adjusted accordingly. In general, it will not be changed, even if subsequent U/S measurements disagree (see below to understand why).

After 27 weeks of pregnancy, a first U/S measurement can only be trusted to plus or minus 21 days! This means that a first ultrasound this late in pregnancy has much less value in calculating the EDC. We generally only use these when we have nothing else to go on, and we know then that we are working with much less reliable information.

A word about “changing” your EDC

Now, these discrepancies (7, 14 or 21 days) explain why we rarely change the due date after the one we arrive at when we are able to take the first U/S into account. When you have your second or third ultrasound, the tech or your Provider might mention that that day’s predicted date is different than your Adjusted EDC. Please don’t take this too seriously – unless the discrepancy goes beyond the limits mentioned above, we will probably not “change” your EDC.

If, at any time, you have questions about your due date, do not hesitate to ask us. We will do our best to explain it, and, as necessary, be glad to re-evaluate and show you all of the calculations we have made.

The baby doesn’t know or care what we say about your due date. All we are changing, when we do change it, is our best guess of what your due date actually is. We will not know when the baby will actually come until it is born.

When Should Your Mother (or Aunt, or Sister) Fly in from Portland?

Based on what we’ve discussed so far, you can see that this is a tough, basically unanswerable question. Outside of serious medical questions, this is one of the toughest problems we deal with. We wish we could give you a good answer. We think that the best advice is for her to come when you find out you’re really in labor and stay until the baby is sleeping through the night, but we know that this is not always possible. Suffice it to say that many of us have a lot of kids, and we’ve rarely been able to time grandma visits well in our personal lives.

Summing It Up

However, it is important that we – you and your doctors - do the most thorough and careful job we can in making that best guess. If you go into labor early, if you’re in a car or other accident during the pregnancy, or if a serious illness or other condition develops, it can become extremely important to know as much as is humanly possible about how far along you are. Many important decisions, including the management of problems late in pregnancy, the timing of inductions and of scheduled Cesarean sections, how we handle threatened preterm labor, and so on, require the best EDC possible. Accurate records and early pregnancy care, including U/S at the appropriate times in the pregnancy, can help to make things as safe as possible for you and for your baby.