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Estimating the Estimated Due Date

Author // Trina Hampton

“I’m due on May 27th!” I was told enthusiastically by a friend who just found out she was pregnant. It took more than a little effort to mask my cringe and share her joy. Little did she know that within hours of her positive pregnancy test, she had already given in to one of the biggest misnomers of pregnancy—the “due date,” also referred to as the EDD, for estimated due date. This is the very first thing to be determined once a pregnancy has been confirmed. On the outset, this seems like a reasonable practice. Parents want to know when to expect their baby, and healthcare providers need to have a time line with which to measure the baby’s growth and well-being.


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Unfortunately, obstetric management today seems to have overlooked the estimated portion of the EDD. Focusing on an EDD can set a woman up for often-unnecessary fetal well-being tests, unnecessary induction, and the increased potential for serious risks during childbirth. One of the first things that will make an EDD inaccurate is the method by which it is calculated, Naegele’s Rule, which is almost universally used in the United States. Dr. Franz Karl Naegele, who practiced in Germany in the 1850s, determined that pregnancy lasted 10 lunar months. His calculation assumed pregnancy lasted around 280 days from the first day of the last menstrual period (or 266 days from ovulation, which would be marked as day 14). This method is faulty on at least two points. One, his method was not based on any scientific fact, only his personal observations within his own practice. Two, few women have textbook cycles of 28 days long, with ovulation on day 14.

A more thorough study performed by R. Mittendorf in 1990 found that pregnancy will last closer to 274 days. Reading further into this study also shows us that multiparas (women with one or more live births) and non-white mothers will have pregnancies lasting closer to 269 days. This shows us that Naegele’s Rule establishes a due date that is a full 3–8 days shorter than what more recent and reliable studies indicate. It does seem odd to rely on a method so antiquated.

Another reason that the traditional calculation for the EDD is inaccurate is the assumption that ovulation, and therefore assumed conception, always occurs on day 14 of the cycle. Few women, and surprisingly few healthcare providers, understand the finer points involved in the conception process. Ovulation can take place as early as the seventh day, or as late as the 20th to 30th day of a cycle, and in some cases even earlier or later than that. Actual conception does not necessarily occur on the day of insemination, either. Healthy sperm can survive for up to five days in fertile quality cervical fluid. This can add another potential week into the estimate. Suppose a woman’s ovulation gets postponed because of a stressful event or because she’s coming off the birth control pill. If ovulation takes place on the 37th day of that cycle, and she conceives at that time, her EDD would be off by a full three weeks. In this situation, a healthcare provider may have concern that the baby is small for gestational age (SGA). This assertion can lead to biological profile tests, concern about placental function, and finally the induction of a baby thought to be developing improperly. All of these carry potential risks, and all stem from the fact that the baby was conceived three weeks later than traditional thinking supposed.

Unfortunately, not many women know when they ovulate. This can be determined during a woman’s monthly cycle by practicing a temperature charting method of birth control, such as the Fertility Awareness Method (FAM). Ultrasound measurements are also used to determine or confirm a due date. Early cell generation occurs at roughly the same speed. All embryos develop at about the same rate until around 6–8 weeks gestation. Beyond that time frame, individual genetics set in and the fetus will grow at its own rate for the rest of its life. An ultrasound performed in the first trimester, measuring the skull size, femur length and distance from crown to rump, will be the most accurate for confirming a due date. Most ultrasounds, however, are usually done at 16–20 weeks gestation to better facilitate examination of major organs and determine the gender.

The later in pregnancy an ultrasound is done, the less useful these measurements become in regard to determining age. There is always a variable within 10 days of the last menstrual period date, and this should not change unless ultrasound measuring is off by more than at least 14 days. As with all mechanical procedures, accuracy is dependent upon the skill of the clinician, the equipment being used and interpretation of the readings. Ultrasounds done after 30 weeks are much less reliable for fetal size, and should not be considered for estimating gestational age.

The mere thinking that there is one magical day that pregnancy should end, and every other day is either early or late, is the largest falsehood of all. It is also the most prevalent. No matter how often we are told that the baby can come “any time within two weeks, either way,” it still seems to get forgotten in the end. A healthy baby born one week “late” isn’t necessarily late, it comes precisely when it’s ready to be born. The human body, both the mother’s and the baby’s, grows and develops in its own time.

No doctor in the world can predict when a baby will crawl or walk or get his first tooth, so it is unreasonable to think that a doctor can predict the very day a woman should go into labor. Now, you might not find a doctor who will outright say that they expect a baby to be born on the assigned date of delivery, but it’s not at all hard to find a doctor that suggests induction within days of passing it. This is an outright indication that a doctor believes in the soundness of a due date. We have become comfortable telling moms that a baby born any time after 36–37 weeks gestation can be expected to be healthy. Unfortunately at the end of an uncomfortable pregnancy, most women are all too eager to hope for a birth soon after that. They have little tolerance for waiting another four to six weeks for labor to begin spontaneously.

There is no time in our society when being late is acceptable. We are a people who need to be scheduled and organized. This puts unnecessary guilt on a mother, who begins to feel as though she’s inconveniencing the people around her by making them wait. This guilt leads to insecurity, which undermines a woman’s confidence, making her request or accept an induction for convenience. The most important thing is for a woman to embrace the idea that her baby can come at any time during the last weeks of pregnancy. Birth professionals can promote this by assuaging the concern that her baby will grow “too big,” informing her that labor pain will be easier to cope with when it begins spontaneously, and reassuring her that she is not inconveniencing anyone with the length of her pregnancy. The last days of carrying your baby within you should be cherished.

You might be wondering how it worked out for my friend. She was induced a full three weeks before her estimated due date. Her doctor declared after cervical examination that the baby was “ready,” and she never questioned the sudden induction. Thankfully, her small baby was born healthy.


Pathways Issue 42 CoverThis article appeared in Pathways to Family Wellness magazine, Issue #43.

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