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Due Dates

Easy Steps to a Safer Pregnancy - View e-book or Download PDF - FREE!
An interactive resource for moms on easy steps they can take to reduce exposure to chemical toxins during pregnancy.

Other excellent resources about avoiding toxins during pregnancy

These are easy to read and understand and are beautifully presented.

See also:

Subsections on this page:


This Pregnancy Date Calculator is my favorite for going from LMP to due date and vice versa.  (And they don't require a complicated email feedback or make you sign up for weekly reports.)  However, their week-by-week charts are off by a week from the standard midwifery/OB calculation, which is completed weeks and days.

Length of human pregnancies can vary naturally by as much as five weeks - 8/6/13  [Ed: I'm still amazed that researchers aren't looking more at genetic influences.  I live in the San Francisco Bay Area, which feels like the melting pot of the world.  As a a general rule, babies with genes from very cold climates need to stay inside longer, and babies from warm climates come a little earlier.  So, what's a midwife to do?!?  My philosophy it to ensure the mother doesn't have subclinical blood sugar issues, which can delay the onset of labor, and to prepare the cervix and uterus to go into labor promptly once the baby throws the switch.  This can be done with herbs or other approaches.  And for moms who naturally go longer, you still need to provide regular monitoring of the mom and baby.  And do a good gestational age assessment on the baby, for your own learning and for the mom's history.]

The Evidence on Due Dates by Rebecca Dekker, PhD, RN, APRN - this is an excellent, well-researched article.  I would have liked to see a discussion about genetic variation of due dates, i.e. Asian vs. Scandinavian/Germanic, and the fact that gestational age assessment shows that there are a number of reasons why babies are born later than their genetics would have liked.  In my experience, the main reason for this is elevated glucose levels during the last half of pregnancy.  Some would call this sub-clinical gestational diabetes.

This due date calculator from babycenter.com takes the length of your cycles into account.

Method for Estimating Due Date [ACOG, Oct., 2014] -  . . . one study found a reduction in the need for postterm inductions in a group of women randomized to receive routine first-trimester ultrasonography compared with women who received only second-trimester ultrasonography.

Why we should eliminate the due date: a truth in jest.
Katz VL, Farmer R, Tufariello J, Carpenter M.
Obstet Gynecol. 2001 Dec;98(6):1127-9.

"We currently use flawed calculations to set a woman's due date based on menstrual periods to determine gestational age. We use the estimated gestational age to make management decisions based on our patients' individual needs. This principle is in contrast to our patients' use of dating to set an estimated date of confinement."

When is that baby due? by Henci Goer

"When researchers in the late 1980s followed a group of healthy, white women with regular menstrual cycles, they discovered that pregnancy in first-time mothers averaged eight days longer than this, or forty-one weeks plus one day (2). The average was three days longer than forty weeks in women with prior births. The researchers also refer to other studies suggesting that other races may have average pregnancy lengths that are shorter than white women."

The SalEst™ test is intended to detect and measure by enzyme-linked immunoabsorbant assay (ELISA) technology the level of salivary estriol in pregnant women.

The device is indicated for use as an aid in identifying risk of spontaneous preterm labor and delivery in singleton pregnancies. The device can be used every 1 to 2 weeks from gestational ages 22 to 36 weeks.

Detailed Paper about PostDates

DATING OF PREGNANCY - A position statement from the Wisconsin Association for Perinatal Care

Figuring Your Due Date - Written for parents

The obstetric wheel is the circular calculator used to figure out the due date as 280 days from the Last Menstrual Period (LMP).  Some people jokingly call this the "Wheel of Misfortune".

Computer software predicts gestation length and risk of preterm birth - Mittendorf-Williams Rule Software

A prospective study comparing the c found it to be twice as accurate as Naegele's rule in predicting gestation.

"A retrospective study by Mittendorf and colleagues (Obstetrics and Gynecology, 1990;75:929-32) determined that white women with singleton pregnancies receiving private care averaged seven days longer gestation than Naegele's rule predicts. A second study by Mittendorf (American Journal of Obstetrics and Gynecology, 1993;168:480-4) concluded that there are several factors including number of previous births, age and race that can be used to determine the length of pregnancy more accurately than Naegele's rule, which does not take other factors into consideration."

I'm not sure how accurate this is, but I found this online: "Mittendorf-Williams Rule. In a study done by Mittendorf in 1990, it was discovered that on average, it takes 288 days from the first day of your last period to delivery in Caucasian women without prior deliveries. Using this method, you will want to take the first day of your last period and subtract three months. Then you add fifteen days if this is your first baby and you are Caucasian. If you are not Caucasian or this is not your first baby, you will want to add on ten days. Some studies have found the Mittendorf-Williams Rule to be more accurate than Naegele's Rule in calculating due date. "

Better way to predict pregnancy term from The University of Chicago Chronicle Nov. 7, 1996, Vol. 16, No. 5

UChicago Press Release - October 22, 1996

Computer Program Predicts Women's Delivery Dates - Medscape report on the Mittendorf-Williams rule software.

Due Dates by Heidi Streufert, CD used to have a dynamic calculator based on Robert Mittendorf's formula.

Predictors of human gestational length.
Mittendorf R, Williams MA, Berkey CS, Lieberman E, Monson RR.
Am J Obstet Gynecol. 1993 Feb;168(2):480-4.

RESULTS: Maternal parity, age, and race were found to be the most important variables determining the length of human gestation. Multiparous women, women aged < 19 or > 34 years, and black women were found to have shorter gestations than primiparous women, women aged 19 to 34 years, or white women.

The length of uncomplicated human gestation.
Mittendorf R, Williams MA, Berkey CS, Cotter PF.
Obstet Gynecol. 1990 Jun;75(6):929-32.
Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts.

By retrospective exclusion of gestations with known obstetric complications, maternal diseases, or unreliable menstrual histories, we found that uncomplicated, spontaneous-labor pregnancy in private-care white mothers is longer than Naegele's rule predicts. For primiparas, the median duration of gestation from assumed ovulation to delivery was 274 days, significantly longer than the predicted 266 days (P = .0003). For multiparas, the median duration of pregnancy was 269 days, also significantly longer than the prediction (P = .019). Moreover, the median length of pregnancy in primiparas proved to be significantly longer than that for multiparas (P = .0032). Thus, this study suggests that when estimating a due date for private-care white patients, one should count back 3 months from the first day of the last menses, then add 15 days for primiparas or 10 days for multiparas, instead of using the common algorithm for Naegele's rule.

Length of pregnancy in African Americans: validation of a new predictive rule.
Mittendorf R, Chorzempa LM, Quinlan MP, Herschel M, Williams MA.
J Natl Med Assoc. 1999 Sep;91(9):523-7.

This study evaluated whether a new predictive rule is more accurate for estimating the length of pregnancy in African Americans than Nägele's rule, the accepted standard. After identifying women in early pregnancy, telephone interviews were conducted to obtain information about 16 previously established determinants of gestational length. Based on these data, a linear multivariate regression model was used to predict an estimated delivery date (EDD) for each mother. In addition, the EDD was determined using Nägele's rule. Later, the actual delivery date was compared with the EDD predicted by the new rule and with the EDD predicted by Nägele's rule. Each pregnancy was assigned to its better prediction group, either the new rule's group or the Nägele's rule group. Fifty-seven pregnancies were identified prospectively and monitored. The new rule predicted the actual delivery date more accurately in 66% (37/56) of pregnancies, Nägele's rule was a better predictor in 34% (19/56) of pregnancies, and both rules were equally accurate in predicting the delivery date for one pregnancy. The new rule was more precise than Nägele's rule (P = .022) when the binomial distribution was used. When using the linear regression model rule, a more accurate EDD can be determined for African-American women. Moreover, it is possible to predict the risk of preterm delivery (those occurring > 3 weeks earlier than the EDD).

Early Due Dates Research Was Flawed

I remember a workshop/address by Barbara Katz Rothman at one of the MANA conferences in the early 80's and she did a retrospective examination of the 40 week due date and discovered that the research that was done to determine this "fact" was conducted during the early 50's when most women were weight restricted (told only to gain 15 lbs or so) and salt restricted. Her point was that perhaps the data itself was based upon a sample that was not "normal" and that the research should be redone with women who are encouraged to eat health. She herself thought that pregnancy was more likely 41 or 42 weeks. I think this idea was then published in one of her books, but i don't have all of them, it might be a place to start looking though. 

Anne Frye - Holistic Midwifery, Mittendorf study ( OB/GYN Vol. 75, No. 6, June 1990, pp.929-932) and offers that formula for calculating EDC. The conclusion was that length of pregnancy from first day of LMP in primips is 288 days and for multips, 283 days. The formula is: Primips: LMP - 3 mo. + 15 days (+ or - days more or less than 28 if cycle length differs) Multips: LMP - 3 mo. + 10 days (ditto)

Frye, "Holistic Midwifery" pp. 364-7 supports 4 or 5.

Wood's method:
nullips: LMP + 1 year - 2 months - 14 days, +or- days cycle varies from 28 days
multips: LMP + 1 year - 2 months - 18 days etc . . .
Nichols, Carol Wood, "Postdate Pregnancy, Part II: Clinical Implications," J. of Nurse-Midwifery, Vol. 30, No. 5, Sept/Oct., 1985, pp. 259-268.

Mittendorf's Study
31 nullips, 83 multips found first pregnancies lasted on average 288 days and multips 283.
Nullips: LMP -3 months + 15 days (adjust for variation in cycle length)
Multips: LMP -3 months +10 days etc . . .
Mittendorf, R. et al., "The length of uncomplicated human gestation," OB/GYN, Vol. 75, No., 6 June, 1990, pp. 907-932.

See also Enkin et al "Guide to Effective Care . . ." which addresses at what point induction is beneficial (not before 2 weeks, I think).

The country folk used to say to remember the state of the moon during your last normal menstrual cycle. Your baby will be ready to come when the tenth moon has passed (280 days). Works about as accurately as anything else!

But the 280 days number assumes a moon cycle of 28 days.  It's actually 29.53 days, so that 10 moon cycles is really 295 days, rather than 280.

Each item taken separately is hopelessly inconclusive, but when all are put together I get a really clear picture of how far along the pregnancy is. I kind of figure out where each would be if that was the only clue, throw out any that are way off, and sort of average it from there. So far I have been pretty accurate and don't worry about waiting, but I do have a fear that one day I will deliver premature twins at home...

These are the parameters that we use for agreement between LMP and sonogram dating.

If good menstrual dates, stay with LMP.

Inital EDD calculations
Lmp ___/___/___        =edd___/___/___
initial ___/___/___ =___wks=edd___/___/___
u/s ___/___/___ =___wks=edd___/___/___
Int.edd  ___/___/___   evaluated by _________
Quick        ___/___/___+22 wks=___/___/___
FH at Umb. ___/___/___+20 wks=___/___/___
FHT/Fescp  ___/___/___+20 wks=___/___/___
Final Edd   ___/___/___   by________________

32 - week edd-uterine size concordance +/- 4 cm is indication for US

Recently there was a thread about a different way to estimate a moms due date ( not the go back 3 months add 7 days one) but one that adjusted for moms who have irregular cycles.

Does anyone remember this and/or know where I can find more info ?

What I was taught was to add 2 days for everyday over the 28 day cycle. For example, if a woman has a period every 30 days, you would add 4 days to the due date figured on the 28 day numbers. Does that make sense?:) I have found it to work pretty well. It works well the other direction too. For women that have shorter cycles.. Just subtract 2 days for every day under 28 days.

Due dates are a little-understood concept.

Truth is, even if you know the exact date when you ovulated, you still can only estimate the baby's unique gestational cycle to about plus or minus two weeks. I have heard reports of doctors saying the following:

"Most healthy women with normal pregnancies carry the baby for eight days past their "due date". I like to induce at seven days past." I don't know how they manage to say this with a straight face, but . . .

Fetoscope Detects Fetal Heartbeat at 13 Weeks

When I was pregnant with my youngest I had been attending births for about a year and a half so I was a bit compulsive about heart tones. Every morning during the early weeks of my pregnancy I would lay in bed massaging my belly and listen for the baby's heart tones with my delee fetoscope. I first heard them at 13 weeks and continued to hear them daily. It got to be a ritual for me and I treasured the quiet time with my babe, as my big kids soon learned that they had to be quiet while I listened and they eventually got bored and went off to play. At the time I was average weight, slimmish might be a stretch.

I have been lucky to hear early fht's with a fetoscope on a few other moms, once at 13 weeks and once at 15 but they've been the exception.

[from a mom]
When someone presses for a date I just ask them a question back: "Why do you want to know?" This helps to open discussion on what I believe about arbitrary due dates, or ends the discussion conveniently short. And I maintain my privacy and integrity for this childbirth.

My main motivation for doing this is that I have decided not to get hung up on an arbitrary due date so I can relax til whenever the baby does come.

Around here, the midwives with OB back-ups are restricted in how far they can go post-dates with the OB's approval. It is part of the agreement between the OB & the midwife. I went 21 days over with my #3. The OB's comfort level was only 14 days overdue. My midwife got around that by telling her back-up that my conception date was fuzzy(it wasn't tho). I still had my homebirth and all were happy.

None of the OBs around here will even say the word "backup" in the same sentence as "midwife", but there is still a "standard of care" that puts a midwife in a pretty precarious position after 2 weeks past "due date".

Sometimes I wish all the moms (especially first-time moms) would come in with "fuzzy" due dates. The most defensible way to do this is for the mom to report her LMP as a week after it really was, but keep a personal diary that lists the true LMP.

Then, in the very unlikely event that the baby comes early, and is really after 37 weeks, but appears to be between 36 and 37 weeks by fuzzy dates, the mom can suddenly recall that she may have been confused on the dates, she checks her diary and voila, she's after 37 weeks.

All of this assumes that the parents have educated themselves on the issues of due dates and postdates and are willing to take responsibility for this.

I especially recommend this strategy to women whose mothers always went several weeks past their due dates or who have had previous postdates pregnancies.

In a world where providers were focused on providing the best possible care instead of focused on providing the most legally defensible care, this wouldn't even be an issue. As it is, I see so much anxiety because just about everyone seems to go postdates. Kind of like Lake Wobegon, where the women are strong, the men are good looking, and the children have gestations that are all longer than average.

I was wondering what happens when the midwife measures the fundal height. If the LMP was offered a week later - would that be off? I didn't do that but I decided that I was going to suddenly tell them I was off a week when I double checked in case I went past my due date. I was afraid in the beginning bc of the measurements - not knowing.

A cm is a very small measurement when you're talking about such a crude measurement as "fundal height". Technically, it's the distance from the top of the mom's pubic bone to the fundus, i.e. the top of the uterus. There is so much imprecision in this measurement that it's laughable. It's to help give you a rough idea of whether the uterus and the baby are growing more or less properly without any sudden changes that might indicate a sudden change in the amount of amniotic fluid.

During the first half of the pregnancy, most practitioners just note a crude approximation, i.e. fundus halfway between the pubic bone and umbilicus, or at umbilicus.

During the second half of the pregnancy, they may start measuring, but the measurement depends on the weeks gestation, the number of babies [grin], the amount of amniotic fluid, the extent of engagement of the baby's head and the baby's position. Babies do stretch out and curl up, after all.

A single centimeter is very, very small among all those factors.

The "rule of thumb" is that the fundal height should measure the same as the weeks of gestation. Most practitioners know the number of weeks before they measure, and it turns out that the measurement usually equals the number of weeks. [grin]

Really, if it's off by much, you double check, assess fluid, and maybe make a note to check carefully in the future.

But it's a very approximate measurement.

I doubt that two practitioners at the same time or the same practitioner at two different times in the same day would get the same exact results.

Two weeks off could possibly raise alarms, as there are other factors related to the weeks gestation, e.g. when the mom feels movement, when you can hear the baby's heart with a fetoscope or a Doppler. And if you have an early ultrasound specifically for dates, it has a much greater degree of accuracy. But most practitioners won't "change your dates" if the discrepancy is small, say, a week. [grin]

Practices vary in their policies, of course, but even hospital midwives in the U.S. often have to transfer care to the OB at ten days past due date. What a lot of pressure to put on a first-time mom, who's only got a 60% chance or so of making it by that time.

Yes, there's something very wrong with the system when being truthful with your caregiver is likely to increase your problems.

Baby's Sex

Male gender predisposes to prolongation of pregnancy.
Divon MY, Ferber A, Nisell H, Westgren M.
Am J Obstet Gynecol. 2002 Oct;187(4):1081-3

CONCLUSION: Male gender significantly predisposes to the prolongation of pregnancy to the extent that, by 43 weeks of gestation, there are 3 male deliveries for every 2 female deliveries.

About Baby's Sex and Length of Gestation

Ah, I finally found out why girl babies tend to come earlier than boy babies.  Estrogen is responsible for facilitating the uterine recruitment of oxytocin receptors during pregnancy.  A pregnancy carrying a girl baby has more ambient estrogen than a pregnancy carrying a boy baby. This means that a uterus carrying a girl baby will be ready to go into labor sooner than if the baby were a boy.

Girl Twin May Determine Pregnancy Length

This seems somewhat odd because the common wisdom among midwives is that girls come earlier than boys.  (I think this is supported by the research but don't have references hand.  Maybe girls somehow "normalize" length of pregnancy, and since twins tend to come early, the normalization in that case is to extend the pregnancy, whereas in a singleton pregnancy, normalization is to come slightly earlier.)

Gestation Length in Elephants Longer for Males?

About Elephants - "The gestation period is between nineteen and twenty-two months. Periodically, it is noted that male infants typically have a slightly longer term than females."

How long after mating do elephants produce their young? - "Exact knowledge on this subject is not available. Estimates of the period of gestation in elephants range all the way from eighteen to twenty-two months. Dr. William, M. Mann, director of the National Zoological Park, says the gestation period in elephants is "about nineteen months, although there are more or less authentic records of twenty-one months." According to Raymond L. Ditmars, curator of mammals at the New York Zoological Park, the period of gestation in the Indian elephant is 641 days, which is considerably more than twenty-one months. There is no evidence to support the belief held by the natives of India and Burma that the gestation period is longer for a male calf than for a female."

Genetic Heritage

In considering the effects of heritage, it's important to remember that ethnic heritage includes significant factors such as diet, which will change as families move from one area to the next.  Genetic heritage won't change with migration, but it's hard to separate these factors.

My personal observations are as follows:

Shorter gestations - Asian, Semitic, Hispanic, Mediterranean (a week or so early with first)
Average gestations: Celtic, Russian or French (right around the due date)
Longer gestations - African, Germanic or Scandinavian (2-4 weeks past the due date)

Postterm delivery predicts subsequent late birth
Publish date: Apr 19, 2007

Women with a history of post-term birth have a nearly twofold risk of a subsequent postterm birth, suggesting that genetic or other common factors may influence the likelihood of abnormal parturition timing, according to a report published in the March issue of the American Journal of Obstetrics and Gynecology.

Louis J. Muglia, MD, PhD, of the Washington University School of Medicine in St. Louis, and colleagues evaluated data from the Missouri Department of Health on 368,633 births, 7.6% of which occurred after 42 weeks' gestation.

Independent of race, the researchers found that mothers with an initial postterm birth had a higher risk for a subsequent post-term birth than those who did not (OR, 1.88). The investigators also found that the risk of postterm birth was positively associated with maternal education of less than 12 years (adjusted OR, 1.51) and a maternal body mass index higher than 35 kg/m2 (adjusted OR, 1.23). Black women had a lower risk for all or recurrent postterm births, the report indicates.

"In contrast to preterm birth, in which a variety of contributors such as drug use, infection, and placental abruption could precipitate labor, the failure to enter labor may define a more homogenous physiologic condition regarding the clock mechanism that times the duration of gestation," the authors conclude.

Kistka ZA, Palomar L, Boslaugh SE, et al. Risk for postterm delivery after previous postterm delivery. Am J Obstet Gynecol. 2007;196:241.e1-e6.

[Ed: P.S. Even though they accept the possibility that the length of a gestation might be genetic, they still want to call it "abnormal parturition timing" instead of normal biological variation.]

Variation in rates of postterm birth in Europe: reality or artefact?
Zeitlin J, Blondel B, Alexander S, Bréart G; PERISTAT Group.
BJOG. 2007 Jul 6

Conclusions These results suggest that practices for managing pregnancies continuing beyond term differ in Europe and raise questions about the health and other impacts in countries with markedly high or low postterm rates. Some variability in these rates may also be due to methods for determining gestational age, which has broader implications for international comparisons of gestational age, including rates of postterm and preterm births and small-for-gestational-age newborns.

[Editor: Maybe people in the northern countries evolved to have longer gestations and bigger babies to protect them from the cold weather?]

Typical Pregnancy is Now Only 39 Weeks - Late Preterm Births Increase [3/23/06]

This article was about babies born at 34 to 36 weeks, which they're calling "late preterm" or "near-term" births.  It's odd that they then just gave the average gestation length without numbers about standard deviation or percentages occurring in each week!

You would think that the statistical analysis would have considered the effects of induction and teased those out. It's also odd that nobody seems to pay attention to the "gestational age assessment" scores when they do these studies.

In any case, I'm wondering if the increase in populations with shorter gestation length (Asian and Hispanic) might not be responsible for shortening the average.

Does a first trimester dating scan using crown rump length measurement reduce the rate of induction of labour for prolonged pregnancy? An uncompleted randomised controlled trial of 463 women
DJ Harrington,a IZ MacKenzie,a K Thompson,a M Fleminger,b C Greenwooda
BJOG: An International Journal of Obstetrics & Gynaecology
Volume 113 Page 171  - February 2006

Conclusions Acknowledging the reduced numbers recruited for study, it is concluded that there is no evidence that a first trimester ultrasound dating scan reduces the rate of induction of labour for prolonged pregnancy and may result in a more expensive healthcare strategy.

[Ed.: I think researchers are looking at the wrong issue.  They're looking to pinpoint the date of conception based on the assumption that all babies grow at the same rate and will reach their ideal term date exactly 266 days from conception (280 days from LMP).  This particular study is in line with a lot of other studies that assume that babies come "late" because the women were wrong about the date of their last LMP or the date of conception.  My recommendation would be for researchers to verify that different babies have different ideal gestational lengths - this kind of study would get a good early date (by ultrasound if necessary, or by blood HCG for women who wish to avoid ultrasound), and then do a very thorough and precise gestational age assessment at the birth.  This is the key factor missing in all of these dating studies - nobody seems to be doing a good gestational age assessment.]

Risk of recurrence of prolonged pregnancy. [Full text]
Olesen AW, Basso O, Olsen J.
BMJ. 2003 Mar 1;326(7387):476.

Women had a reduced risk of recurrent post-term delivery if they changed partner between pregnancies (table). We believe that this is a new observation. This result suggests that the timing of birth may, in part, be determined by paternal genes.

The rapid responses to this article contain additional information, including a contribution from Ronnie Falcao (aka Veronica Falcao), the editor of these Midwife Archives.

An estimate of the tendency to repeat postterm delivery.
Olesen AW, Basso O, Olsen J.
Epidemiology. 1999 Jul;10(4):468-9.

Postterm delivery among African Americans, Mexican Americans and Whites in Chicago.
Collins JW, Schulte NF, George L, Drolet A.
Ethn Dis. 2001 Spring-Summer;11(2):181-7.

CONCLUSIONS: We conclude that African Americans and Mexican Americans have greater postterm delivery rates than do Whites; however, commonly cited individual and community-level risk factors account for most of the disparity. [Ed. - I interned at a birth center in El Paso, Texas, serving a mostly Hispanic clientele.  They seemed to come mostly right around their due date.  If I get a chance, I'll go back and do some statistics.]


One of my clients told me that winter babies come a few days later.


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