Estimated Due Dates

DueDateWe live in a culture where we seek to make everything measurable and predictable. Just as we might expect our favorite TV show to come on at 7pm sharp, our work day to end at 5pm on the dot, and so on, many of us want or expect our pregnancies to begin and end in a predictable way. Yet, with birth (and death), there is the element of mystery, of the unknown, to which we must humbly surrender. There is also the fallibility of the methods we have in estimating birth dates. While the great myth that babies are supposed to be born on their due dates pervades our cultural consciousness, this expectation or assumption is at the very least misleading, often causes undue emotional burdens on birthing families, and at its most serious holds the potential to cause serious harm to a woman and her child. Focusing too much on a due date, particularly in conventional maternity care, can create a situation that increases the likelihood of unnecessary fetal testing, unnecessary induction and cesarean, and other serious risks, not to mention undue mental and emotional stress for mama and her partner.

We love the saying “babies are born on their birth dates not their due dates.” Babies are born when they are ready to be born and when all those factors that influence birth timing converge on a specific given day. We’ll talk below a great deal about the scientific understanding (and lack thereof) of birth timing, but even without these details, it is important to remember that babies are born on their birth dates not their due dates! and that there is a great range of what is considered normal gestational duration for the human being.

Naegele’s Rule: the not-so-gold standard in due date estimation

Due dates are most widely calculated based on something called Naegele’s Rule. Trouble is, use of this rule is considered outdated and inaccurate for many women.

This rule was developed by an early 1800s German doctor named Franz Karl Naegele who concluded, based on his personal observations (not any methodical research), that pregnancy lasted 10 lunar months, or 40 weeks. His calculation assumes that pregnancy lasts 280 days from the first date of the last menstrual period, the LMP, or 266 days from ovulation, which he deemed always occurs on day 14 of a woman’s 28 day cycle.

Naegele’s Rule follows this formula: (LMP + 7 days) – 3 months = Due Date

This antiquated method is the standard in determining due date, yet only 3 to 5% of babies are actually due on their due dates. There are some major flaws with this nearly universal way of determining due dates:

  1. It assumes that pregnancy lasts the same duration for all women.

The duration of pregnancy varies based on many factors including:

  • Whether a women is having a first or subsequent baby (some estimates suggest pregnancy is an average of 5-10 days longer for first time birthing women)
  • The race of the mother. For example, one researcher noted that black women tend to have pregnancies shorter by 8.5 days compared to white women of similar socioeconomic status.
  • Number of babies. Women carrying twins, for example, have a shorter pregnancy on average, than women carrying a single fetus.
  • Nutrition
  • Substance abuse
  • Mother’s age
  • Mother’s size
  • Mother and baby’s health

Experts also submit that additional factors, which we don’t fully understand, may also be at play.

  1. It assumes that reports of one's last menstrual period are 100% accurate

Naegele’s rule depends on the accurate recall of the first day of a woman’s last menstrual cycle. While many women are good at such recall, there is room for error here. Inaccurate memory, the possibility of interpreting post-conception spotting as a period, and unrecognized pregnancy loss can all alter what might be reported as one’s last menstrual period.

  1. It assumes all women have 28-day cycles and ovulate on day 14.

This method of calculation assumes that all women have a 28-day cycle and all ovulate on day 14 of this cycle. But we are not machines. Many women have cycles that do not match this 28-day trend. Recent discontinued use of oral contraceptives, current use of other medications, stress, travel, as well as other physical and lifestyle factors can all impact the length of a woman’s cycle and when she ovulates.

This method of due date calculation assumes a woman ovulates on day 14. This too is not often the case. Ovulation can take place as early as the 7th day and as late as the 20-30th day of a woman’s cycle.

Many women are unaware of when they ovulate, but some women are attuned to/understand how to determine ovulation based on regular charting of their waking basal body temperature, cervical fluid changes, and other indicators. (Learn more about the fertility awareness method/FAM online or in the book Taking Charge of your Fertility by Toni Weschler). Ovulation dates may help inform a due date calculation, as can known dates of intercourse. However, many practitioners prefer to rely on LMP, as reported ovulation dates and intercourse dates carry a margin of error in many provider’s minds.

  1. It assumes conception takes place during ovulation.

While ovulation is the time of heightened fertility, conception does not necessarily take place on the day of ovulation (or even the days just before or after). Healthy sperm can survive in fertile quality cervical fluid for up to 5 days, which can change the date of conception and alter a due date by several days.

Mittendorf’s Rule

In the late 1980s, a doctor named Robert Mittendorf reviewed the records of 17,000 births (his practice was composed primarily of second generation Irish-Americans). He found that most healthy, white, private-care, first time birthing women with regular menstrual histories experienced pregnancies lasting an average of 288 days from LMP to birth, (or 274 days from ovulation to birth) a full 8 days longer than Naegele’s Rule postulates.

Thus, Mittendorf’s rule follows this formula: (LMP – 3 months) + 15* days = Due date

*use 10 days if a women is not white and/or if she has given birth before

Again, this supposes that a woman knows her LMP and that her cycle is 28 days with ovulation on day 14. It also doesn’t seem to suggest any definitive information on women not belonging to the same population as was studied by Mittendorf.

Early ultrasound measurements

Early ultrasound (in the first trimester) can also be used to determine gestational age of a fetus. Early cell generation occurs at the same pace in all human fetuses until about 6 to 8 weeks of gestation. Beyond that, individual genetics set in and the rate of growth is unique to that individual.

While ultrasound before week 8 can be used with some accuracy (some say

greater accuracy than with Naegele’s rule), one researcher estimates that ultrasound dating accurately predicts the date of birth in less than 5% of all pregnancies, with delivery within 7 days of this estimated due date occurring in only 55% of pregnancies. Not a particularly reliable alternative, some would argue. (And ultrasound exposure may pose additional risks to a fetus.)

It should also be noted that second and third trimester ultrasounds are not accurate in determining length of gestation and can also be inaccurate in determining fetal size.

Other informants of gestational age

Other sources of information, while less precise, can help us learn about gestational age including physical estimation of baby’s size by a qualified care provider, quickening (or the onset of fetal movement felt by the mother), and detection of fetal heart tones.

Wide range in normal development

Even if one were to know with total accuracy the first date of LMP, the date of conception, and early fetal age, they still may not be able to predict with accuracy the date on which baby will be born. This is because there is a normal range of gestational age in humans that spans several weeks (generally considered 37 to 42 weeks). Just as every baby doesn’t sit up, sprout their first tooth, or walk at the exact same age, babies don’t all require the exact same amount of time in the womb. Some take more and some take less, and science hasn’t figured out all the reasons for this.

Problems with inaccurate due dating

Medically speaking, due dates are used to determine when prenatal tests should best be performed, what ranges of results are considered normal, whether baby is growing at a healthy rate, and whether a baby is premature or postmature. While pregnancy dating may help us gain insight into the health of baby, inaccurate dating can cause a number of problems.

Certain prenatal tests must be done during a certain window of time during pregnancy. Inaccurate pregnancy dating can result in inaccurate prenatal tests. For example, fetal heart rates become measureable at a certain week. If a fetus is thought to be older than is it, and fetal heart rate does not register as expected, a problem may be suspected in a case where there is no problem. This can lead to unnecessary testing and concern about a healthy fetus. The AFP test/triple screen test is another test that must be done within a certain window of gestational time. With this test too, inaccurate dating can yield unreliable and misleading test results.

Inaccurate dating can lead to unnecessary induction and/or cesarean, especially in cases where a care provider relies too heavily on a due date calculation to make medical decisions. Labor is induced in over 13% of all US births, with post-date pregnancy (after 40 weeks) being the number one reason given. According to ACOG, 95% of all babies born between 41 and 42 weeks of estimated gestation are born safely without complications. Even after the 42nd week, only a very small percentage of babies have complications due to postmaturity.

Babies can be born premature due to unnecessary interventions, which carries many risks to the health of baby. It is estimated that up to 10% of neonatal intensive care unit admissions are due to iatrogenic prematurity (that is, caused by unnecessary medical interventions to bring labor on before baby is mature). There are ways to reduce the risk of unnecessary interventions, such as amniocentesis and allowing a woman to go into spontaneous labor before intervention. The risk of iatrogenic prematurity and newborn lung disease in infants delivered by elective cesarean before labor begins is 30% compared to 11 percent for those whose mothers go into labor first.

Inaccurate due date calculation or over reliance on this date as THE date baby should ideally be born causes undue psychological burdens on women and their families. So many women feel this tremendous pressure to deliver their babies on or before that magical due date day. Women are often told that baby is mature at 38 weeks and to expect their arrival any time in the next two weeks. When that due date comes and goes, many women feel like something is wrong or they are somehow to blame for not having yet birthed. Many people make so many work or family plans around that magical due date that women can feel additional pressure and a lot of guilt for “inconveniencing” people when birth doesn’t happen exactly when they expect it should.

While we tell our mamas, especially first time mamas, to expect to go over their due date by 7 to 10 days, we know that the overwhelming social pressure (and often physical readiness on mom’s part!) still makes it difficult to relax in those days past the due date. It can help to better understand due date calculation and the truth that there is a wide range of normal. At Health Foundations we are very proactive in watching mamas and babies who go beyond 40 weeks, but that is prudence and not pressure to birth or worry that something is amiss.

Postmaturity risks

While the risk of postmaturity complications is relatively low, it can be helpful to know what these risks are. Postmaturity syndrome is characterized by:

  • Diminished functioning of the placenta
  • Reduced amniotic fluid
  • Large size of baby (or, conversely, sudden fetal weight loss)
  • Increased risk of meconium aspiration (when baby inhales amniotic fluid containing its first stool)
  • Hypoglycemia in baby

There are a number of tests care providers offer to monitor mamas and babies in the 40-42 weeks of pregnancy to diagnose postmaturity syndrome.

While our estimations of due dates are far from perfect, this practice doesn’t seem to be going anywhere and does offer us some value. We think it is most helpful to find a care team that doesn’t regard the due date as this fixed time before which babies should be born. After all, it’s not a deadline (we have plenty enough of those in our lives, don’t we?) it’s a guess date. It may serve many families to think about the due window. It certainly would better reflect the reality that birth presents us.

And remember 100% of babies are born on their birth dates, but only a very small percentage are born on their estimated due dates.