Only about 5% of babies are born on their estimated due date! Is the common way to calculate the date of conception and the due date accurate? And why does it matter? Pregnancy lasts roughly 40 weeks, with gestational age typically calculated from the first day of the woman’s last menstrual period, not from the actual date of conception. Unless a woman charts her fertility and knows her time of ovulation, the estimated due date can be off by a week or more. This error could lead your doctor to recommend an induction when it is not needed.
How a “past-due” pregnant statistician resisted her doctor’s advice to induce
Dr. Talithia Williams is an Associate Professor of Mathematics at Harvey Mudd College whose experience includes research appointments at the Jet Propulsion Laboratory (JPL), the National Security Agency (NSA), and NASA. In her popular TEDx Talk[i], she describes how at 41 “official” weeks of gestation, her doctor strongly suggested that she be induced, claiming that by waiting, her risk of stillbirth would double.
Many women, if uninformed, would very likely comply with the doctor’s request, even when this meant other risks[ii]. In fact, induction of labor increases the risk for the use of forceps and vacuum extraction, or even a c-section. It also increases the chances of your baby being admitted to the neonatal intensive care unit, your baby having jaundice or being premature.
But because Dr. Williams cares about data, she asked for more information and found out that by waiting, the actual risk of stillbirth went from only 0.1% to just a 0.2% chance. Still, one could say, it’s such a terrible event that it should be avoided by any means. But Dr. Williams had another critical piece of information which was her ace in this conversation: what if the due date were wrong?
In fact, she shows in her talk how, by charting her temperature, she had been able to know precisely the date of conception of her first child. And so, she walked out of the hospital confidently, having saved herself from a risky medical procedure that was totally uncalled for.
Why accurate knowledge of the due date is critical
The American College of Obstetricians and Gynecologists say that “accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative,” and that “an accurately assigned estimated due date (EDD) is among the most important results of evaluation and history taking early in prenatal care.” Your EDD determines your obstetric care, and the scheduling and interpretation of certain tests. It determines the appropriateness of fetal growth and drives interventions to prevent preterm births, post-term births, etc.
Pretty much everything about good prenatal health care is based “knowing” when a baby is due or what his/her gestational age is, which, of course, is based on when the baby was conceived!
The traditional way to establish the date of conception is based on the assumption that women have “typical” 28-day cycles, cycles that are used to estimate due dates (EDD) based on last menstrual periods (LMPs). In reality, less than 15% of women have a 28-day cycle. A 2000 study[iv] reports that “in only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines—that is, between days 10 and 17. Most women reach their fertile window earlier and others much later.” It means that 70% of the time, the due date calculation could be off by a week or more!
As in Dr. Williams’ experience, doctors concerned with the higher risks of miscarriage will push for an induction based on the EDD, which is based on inadequate data. The CDC reports[iii] that in 2012, 23.3% of all birth were induced, the majority of which were at term or post-term. But were they really?
How you can know the real due date
It is not impossible or even unlikely for a woman to ovulate on day 21 or beyond, which would put her due date at week 41, not 40, by the traditional way of calculating the EDD. Irregular cycles or even stress can cause ovulation and conception to happen later than the usual clinically estimated window of day 10-17. Ovulation can happen as early as day 7, in which case a woman would be late if she waited until week 40 based on the common definition of the delivery date.
Here is another example, from blogger and birth doula Lindsey Morrow, who writes:
“During my second pregnancy, my last menstrual period was 2/29/2012, which made my estimated due date 12/5/2012, which was wrong, wrong, wrong. Based on ovulation a more accurate due date was 12/13/2012.”
Right now, ultrasound in the first trimester is the most accurate method to establish or confirm the gestational age in conjunction with last menstrual period. But Lindsey Morrow adds: “When I get pregnant I always know how to date a pregnancy and what my true EDD (estimated due date) is. When I go in for an ultrasound and the baby isn’t measuring up to my dates I know something is very wrong.”
Wouldn’t it be great if a pregnant woman could walk into her OB/GYN’s office and tell the doctor how many weeks pregnant she was based on her chart?
She can. By charting, a woman can know more accurately the date of her ovulation and thereby establish a far more accurate delivery due date.
One of the great benefits of fertility awareness methods (FAM) is that they give couples a better knowledge of when ovulation is most likely to occur. For instance, with the symptom-thermal method in which you keep track of your basal temperature, you will be able to interpret the elevation of this temperature as proof ovulation just happened. If the temperature remains high and your periods do not return, you can know with certainty that you conceived and when.
So what can you do? If you are trying to conceive, we encourage you to learn more about how conception happens, find out about charting and FAM. Contact a local or online instructor to find out how it can help you in your particular situation, whether you have irregular cycles, difficulties getting pregnant or even if you have experienced miscarriages. All women should avail themselves of this science because it bears so many promises.
References
[i] Own your body’s data by Dr. Talithia Williams, Filmed February 2014 at TEDx Claremont Colleges.
[ii] 5 Reasons to Avoid Induction of Labor, By Robin Elise Weiss, Ph.D. – Reviewed by a board-certified physician.
Updated September 06, 2016
[iii] Recent Declines in Induction of Labor by Gestational Age, Michelle J.K. Osterman, M.H.S, and Joyce A. Martin, M.P.H. NCHS Data Brief No. 155, June 2014, https://www.cdc.gov/nchs/products/databriefs/db155.htm
[iv] The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study, by AJ Wilcox – BMJ. 2000 Nov 18; 321(7271): 1259–1262.
Written by Gerard Migeon, with research assistance from Emily Kennedy
When this article refers to fertility awareness methods (FAM), or natural family planning (NFP), we are referring to Fertility Awareness-Based Methods, evidence-based methods of cycle charting which can be used as effective forms of natural birth control when learned by a certified instructor.
Yes, this information has been very helpful for me in my own pregnancies. But, I’ve been told that the first trimester ultrasound can be off by as much as a week-is this true?