Estimating the Estimated Due Date
by 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 the joy of my friend. Little did she know that within hours of her positive pregnancy test, she had already given into one of the biggest misnomers of pregnancy - the "due date" also referred to as the E.D.D. for Estimated Due Date. This is the very first thing to be determined once a pregnancy has been confirmed. On the outset, this seems to be a reasonable practice. Parents want to know when to expect their baby, and health care providers need to have a time line with which to measure the baby's growth and well being. However, obstetric management today seems to have overlooked the estimated portion of the E.D.D. As I will discuss, focusing on an E.D.D. is one of the first things that will 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 E.D.D. inaccurate is the very method in which it is calculated. Naegele's Rule is the method that is nearly universally used in the United States. Dr. Naegele, who practiced in Germany in the 1850's, determined that pregnancy that 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 - day 14). This very 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 being 28 days long, with ovulation on day 14. A more recent and thorough study (Mittendorf 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 show us to be more accurate. It does seem odd to rely on a method that is 150 years old.
Another reason that the traditional calculation for the E.D.D. is inaccurate is the assumption that ovulation, and therefore assumed conception, always occurs on day 14 of the cycle. Few women and surprisingly few health care providers understand the finer points involved in the conception process. Ovulation can take place as early as the 7th day, or as late as the 20th-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. If this is the case, then this adds another potential week into the estimate. Suppose a woman's ovulation gets postponed because of a stressful event or perhaps because of coming off the birth control pill. If ovulation takes place on the 37th day of that cycle, and she conceives at that time, her E.D.D. would be off by a full three weeks. In this situation, a health care 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 of this because the baby was conceived three weeks later than traditional thinking allowed for. Unfortunately, not many women know when they ovulate. This can be determined during her monthly cycle by practicing a temperature charting method of birth control, such as the Fertility Awareness Method (FAM.)
Ultrasound measurements are also used for determining or confirming 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 crown to rump length will be the most accurate for confirming a due date. However, most ultrasounds 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 accurate the measurements become. There is always a variable within 10 days of the last menstrual period dating and this should not change unless ultrasound measuring is off by more than at least 14 days. As with all mechanical procedures, accuracy is dependant 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 2 weeks either way" still seems to get forgotten in the end. A healthy baby born 1 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, grow and develop in their own time. No doctor in the world can predict when a baby will crawl or walk or get his first tooth, so it is beyond reason to think that a doctor can predict the very day a woman should go into labor. Now you may 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 passing it. This is an outright indication that a doctor does believe in the soundness of a due date.
We have become comfortable telling moms that a baby born anytime 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. She has little tolerance for waiting another 4-6 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, and is what undermines a woman's confidence making her request or accept an induction for convenience.
The most important thing is for women to embrace the idea that her baby can come at anytime during the last weeks of pregnancy. Birth professionals can promote this by displacing the concern that her baby will grow 'too big', inform her that labor pain is 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 may wonder 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.
Abstract for "The Length of Uncomplicated Human Gestation" by Mittendorf et al. Obstetrics & Gynecology, V.75, N.6, June 1990
Taking Charge of Your Fertility by Toni Weschler
The Thinking Woman's Guide to a Better Birth by Henci Goer
When is that baby due? by Henci Goer