Monday, September 03, 2007

Due Dates

Something I wrote last year...enjoy.

Due dates are a bunch of nonsense. Let me explain why:

1. Only 5% of women go into labor on their due date (In other words, it is wrong for 95%!).

2. The notion that pregnancy lasts 40 weeks from the last menstrual period came from a German physician in 1807 (Hermann Boerhaave). He didn't do any statistical studies on average length of pregnancies--he just came up with this number based on observations. In part, because it was a nice, easy number to remember (10 lunar months). This method of calculating due dates later became called "Naegele's Rule" and is the one that most doctors use today, even though there is no statistical evidence that 40 weeks is the correct average length of pregnancy.

3. Naegele's Rule is based on the last menstrual period and assumes a 28-day cycle. However, many women have cycles longer or shorter than 28 days. Ovulation usually occurs 14 days before the next period (NOT 14 days after the previous), so a woman with a 35-day cycle will ovulate later in her cycle than a woman with a 28-day cycle.

Let me illustrate this with an example: Let's say there are 3 women whose menstrual periods all began on the same day. I'll say Jan 1st for convenience.. They all became pregnant during the same cycle. Naegele's Rule (and most doctors) would give them all the same due date. However, that is flawed:

Amy has regular 28-day cycles. Her 40-week "due date" would fall on Oct 7.
Becky has regular 35-day cycles. Her 40-week mark is Oct 14.
Cindy has irregular cycles, ranging from 30 days to 55 days. Thus, her 40-week mark could be on Oct 9, or Nov 2, or anywhere in between.

4. The average length of pregnancy is not 40 weeks (okay, technically 38 because there are 2 extra weeks thrown in there before conception, but that is confusing to most people so I'll stick with 40). Women gestate babies for different amounts of time. Just like some of us grow faster, hit puberty at different times, or start our periods at different ages, women also grow babies at different rates. For example, in studies done of healthy Caucasian women, the average length of pregnancy was 41 weeks 1 day for first babies, and 40 weeks 3 days for second babies.

Next, you'll remember from statistics class that averages are calculated from a wide range of possibilities. Term pregnancy is generally considered to be from about 37 weeks and beyond. Many women go into labor between 37-42 weeks. However, a substantial minority gestate much longer--often 44 or 46 weeks. The longest documented pregnancy is 52 weeks. Yes, one full year. And you thought "Ten Month Mamas" had it hard!

So, even if you take into account the length of your menstrual cycle, and the fact that average pregnancies are 41 weeks 1 day for first babies if you're Caucasian, that still tells you nothing about what specific day you will go into labor. Hence the 95% inaccuracy rate. It just gives you a 5-6 week (or more) window in which your baby will probably be born.

5. Ultrasound dating is not 100% accurate, and gets worse farther along in pregnancy. 1st trimester estimates are +/- 5 days, while 3rd trimester estimates are +/- 22 days (or a 44-day window of possibility).

6. Way too many women go through unnecessary stress worrying about their due date. No wonder, since 95% of them are wrong!

I got most of my information from Anne Frye's Holistic Midwifery Vol 1, and Henci Goer's The Thinking Woman's Guide to a Better Birth.


  1. Perhaps I should send this to all of my friends who get completely wacky as their "due dates" approach, and keep it for myself in case I get wacky next time, too. :)

  2. How true it is!! I'm cringing at the idea of a year-long pregnancy I'm gonna have to Google that one.

    It also irritates me that everyone refers to pregnancy as lasting 9 months, when it's actually TEN. 40 weeks, 4 weeks per month = 10 months.

    I knew better than to trust an EDD when the first guesstimate my midwives gave me was August 22. Then they changed their minds and said Sept. 8. I wound up giving birth on Sept. 7. :)

    1. Jill, if the EDD is "estimated" at 40 weeks, which includes the two weeks from this hypothetical LMP, that leaves you 38 weeks. And 4 weeks does NOT equal a month since most months are 30 or 31 days. So when you divide 38 weeks by the average number of days in a month, 30.4 days/month, you end up with 8.75 months, NOT 10 months.

      An EDD is just that, an estimated due date to reference your progress through your pregnancy. No one actually thinks the EDD is the date you should deliver on. The EDD helps you determine roughly when quickening should occur, when to perform genetic testing or glucola, stuff like that. We know that the placenta ages and begins diminishing in function as it ages, and amniotic fluid levels peak on average around 36 weeks and then gradually declines. We know stillborns increase in frequency each week after 37 weeks and in decreased fetal movement is appreciated, or elevated BP, or lower AF, then more antenatal testing should be instituted to determine whether the pregnancy may continue or be recommended for delivery. Sure some babies may be delivered early and not end up on ventilators and some pregnancies may proceed to 46 weeks without stillborn, but the EDD serves as a reference to monitor the pregnancy, not as a date for delivery.
      Most good HCP, MD and CNM get an ultrasound at 6 to 8 weeks gestation that like you said IS accurate to +- 5 days. This will correct for women who have less than 23 day cycles or longer than 33 days cycles. The HCP will change the LMP dated EDD to the sono derived EDD and therefore be more accurate.
      Your depiction of the EDD is so adversarial it's kinda funny and counterproductive to educating your readers falsely! Thus making your readers not appreciate its use as a reference that can better inform them of their risks at term.

  3. Well to picks some nits here...a month is longer than 28 days, plus 40 weeks throws in an extra 2 because it counts from LMP, not conception. So basically it's not really 9 or 10. Really, the whole thing about due dates is that it is an estimate, not a "best before" or "expires by" or even "guaranteed to emerge by" date.

    The whole "you will go into labor on September 5th" nonsense is why I didn't give myself, or anyone else, a due date. Instead, I told people a rough estimate (later than I was really anticipating, so I wouldn't be hassled if I went later). Worked like a charm! I was pleasantly surprised because I didn't have one "day" that I was fixing all my hopes on. I knew my exact day of conception and usually could remember what week gestation I was at, but somehow that felt different. Saying to myself "I am 36 weeks gestation" felt way different than "I am due in 2 weeks." One was a statement of fact; the other is a statement of belief that most likely will pan out to be false.

  4. When I was pregnant last year and folks asked me my due date, I told them I didn't acknowledge due dates and why. If they pushed, which they did, I told them my baby would probably come during the month of Ootober. I think 'due dates' totally set women up and docs can manipulate them to get women into inductions and sections.

  5. Rixa, do you know of any studies that examine the length of gestation of different babies by the same mother? Do they tend to 'cook' for roughly the same time for each mom?

    On second thought, how would *they* know when they induce so many women, or schedule sections anywhere after 37 weeks...

  6. Don't know of any studies off the top of my head, although I know it can vary from baby to baby. It seems that gestation length goes through the maternal line--so if your grandma and mom both had 44 week pregnancies, you are quite likely to as well.

  7. I deleted your earlier post because I have seen enough of your comments elsewhere to know that calm, rational discussion will not occur. I want to keep this a non-hostile forum.

    Plus the issue in my post wasn't stillbirth. Your comments were both hostile and irrelevant so I chose to remove them.

    The real issue isn't who is "right" or "wrong." It's who gets to define reality and decide what counts and what doesn't. The power to speak authoritatively, to define what knowledge is orthodox and what is not, is an immense one. Western biomedicine currently claims ownership of authoritative knowledge in our culture.

    Anthropologist Brigitte Jordan, best known for her book "Birth in Four Cultures," explains that authoritative knowledge is “the knowledge that within a community is considered legitimate, consequential, official, worthy of discussion, and appropriate.” It is not the knowledge of those in authority positions, as the term might suggest, but rather a “state that is collaboratively achieved within a community of practice.” Whether or not the knowledge system is “right” or “truthful” is irrelevant; “the power of authoritative knowledge is not that it is correct but that it counts.” A community or society will often have multiple knowledge systems, some of which gain dominance “either because they explain the state of the world better for the purposes at hand (efficacy) or because they are associated with a stronger power base (structural superiority), and usually both.” When one knowledge system becomes authoritative, it often delegitimizes all other kinds of knowing while simultaneously presenting itself as a “natural order, that is, the way things (obviously) are.” People who embrace alternative knowledge systems face strong penalties for their unorthodoxy.

    Debates about home vs hospital birth are so polarized because the opposing sides are really contesting deeply held paradigms. It's not quibbles over statistical data that's at stake. I doubt that the debates will accomplish anything except cement both sides more deeply in their own worldviews. It's like a Catholic trying to prove or disprove Buddhism based on Biblical teachings. You can't use totally different belief systems to prove or disprove each other.

    I very rarely remove comments, so when I do I ask people to respect that.

  8. My cycle is 28 days... to the DAY. Even after my son was born, and I got my period back 2 months later *even with BFing* It came back on the same day as it was expected to come when I was pregnant with him.

    that being said... i don't ovulate until the end of the month. So I conceived my son on the 26th, period was due on the 5th, i didn't get a positive pregnancy test *barely there line* until the 13th.

    Same thing happened this time. I conceived at the end of the month and didn't get a positive pregnancy test until way after my missed period.

    it's very annoying!

  9. oh and my son was born 10 days "early" I'm expecting much the same this time around when my baby comes sometime in feb ;)

  10. My issue with due dates, though, is that the very term implies a level of precision that we simply can't attain. LMP and ultrasound still only give us rough guesses. Unless the woman knows her date of ovulation & conception precisely, we still can't know if she's really reached "40 weeks" or not...That issue is important and has implications for any study about correlating X factor with length of pregnancy.

    There's also a great amount of stress about going even one day past the magic 40 week "due date." If a woman goes into labor the day before, she doesn't freak that she's "early." But if she goes one day over, she'll start telling everyone she's "late" and "overdue." Bizarre.

    1. Again Rixa, how does EDD (ESTIMATED due date) give a certain level of precision. No one uses it as precision and you posting that "they" give it a level of precision is only a scare tactic for women. EDD is an estimate, best accurate with women having 28 day cycles. Early sonos are accurate or inaccurate +- 5 days, second trimester sonos inaccurate +- 11-15 days, and third trimester sonos inaccurate +- 18-21 days. An EDD gives HCP a timeframe to watch pregnancies more closely. Only a naive person or naive HCP believes it is the day you should deliver on.

    2. I wonder where you are getting your information from when you say "no one uses it as precision"? My friend was "due" last week and her doctor is already talking about inducing because the baby is a week late and is (in her words) "only going to get bigger". So actually, many people do seem to believe it is precise including OB/GYNs.

      I don't see how Rixa's post is a scare tactic...she has provided information and left it to the reader to decide whether/how to use this information. I think the example I've given of a doctor trying to influence her patient to induce labour is much more of a scare tactic.

      PS. Rixa I started reading your blog last week and started at the beginning. Not sure when I'll catch up to real time but so far this is great stuff! Thanks.

  11. Forgot to add citations to my quotes from Brigitte Jordan:

    It comes from her chapter "Authoritative Knowledge and Its Construction" pp 55-79 in the book "Childbirth and Authoritative Knowledge: Cross-Cultural Perspectives," edited by Robbie E. Davis-Floyd & Carolyn F. Sargent (Berkeley: UC Press, 1997).

  12. Well, you picked the right nits Rixa, hee hee. I guess what I should have said was 10 lunar months. Not really calendar months.

    It's all just numbers! :)

  13. Hi Rixa, am very interested in your comments both from the point of view of an academic and midwife. I often refer to the concept of the medic's authoritative knowledge which often over rules that of the midwife, and of course, the woman. My clients often gasp when I tell them that I went nearly a month overdue with my son - and he's been trouble ever since! Cheers Sarah

  14. Dr Amy, since you are closely following this blog I suspect you have seen the warning on the margin:

    All content copyrighted. Do not repost or use without permission.

    Do you have permission to post Rixa's blog entry on yours? I see she gave you permission to debate yourself elsewhere, but didn't see she gave you permission to post for her on your blog. Bad blogger form. Tsk. Tsk.

  15. Sarah,
    If you haven't already, you'd love the book "Childbirth and Authoritative Knowledge." It's a collection of scholarly essays dealing with all different aspects of authoritative knowledge during pregnancy and birth.

  16. Isn't gestational age the age from conception vs. the age from LMP. So, if the study says at 41-43 weeks from gestation the still birth rate rises, it's what doctors normally call 43-45 weeks. Am I missing something here?


  17. A certain doctor, who is no longer licensed to practice, sits in her pink mc-mansion and while gazing at Horizons in Sharon, trolls the internet drumming up hits for her blog. She garners her second income in this way, and entertains her bored SAHM mind bickering with anyone who talks about natural childbirth. You would do well to delete anything she posts here.

    A little piece of evidence to bolster your due date is nonsense post, is pointed out by Gail Hart in Research Updates for Midwives 2005. More modern data (not the McClure-Brown 1958 data published in 1963 that above mentioned Dr. relies on) from both 1982 Williams Creasy, and 1987 Eden Sefert, show that perinatal mortality does not rise much until after 43 weeks. NOT 40 weeks.

    Gail writes “The Weinstein, et al., study particularly stands out. This retrospective study compared almost 1,800 reliably dated post-term pregnancies with a matched group that delivered “on time” (between 37 and 41 weeks) Perinatal mortality was similar in both groups (.56/1000 in the post-term and .75/1000 in the on time group). Contrary to popular expectations, the rates of meconium, shoulder dystocia, and cesarean were almost identical. The rates of fetal distress, instrumental delivery and low Apgar were actually lower in the postdate group than in the on-time group. This is only one of several studies showing postdate pregnancies can be safely monitored until delivery. The key to safety is, of course, monitoring.”

    The positive attribute to being hit by the doctor everyone loves to hate, is that your readership will go up!

    Anonymous number 14

  18. Very, very interesting, Rixa. I've been reading your site a while but not commenting, since I haven't yet had children, but I'm trying to study up first like you did. I myself was born 8 days "late" with no problems, but now nearly all of my friends and friends' wives have been railroaded into inductions because they were "late" by a few days. Because of that, the whole due date thing has always rung pretty false for me, and this makes a lot of sense. Thanks!

  19. Oh wow, thank you Anonymous 14, I had no clue about the research in Gail Hart's article -- I will look it up! Kind of excrutiating, how slowly information trickles down to... well, to me. How did I miss this? I was still convinced that 42 weeks is significantly riskier than say 40. Not that *I* would choose to induce, but something to be aware of.

  20. Just arrived in AZ after a long day of travel! Anyway there's lots of blog-o-drama going on about this post and discussion and I am not going to go there. So I am deleting all of the related comments from a certain poster and moving on.

  21. Rixa,

    She has all ready put up a fuss about you deleting some of her other comments. Funny how that works...on her site it is fairly common for posts to be deleted and even more common for a poster to be banned. It happens over and over.

    Of course, I am sure Dr. Amy's comment would be that she *only* deletes posts that are personal attacks, not posts that are information based (or claim to be).

    But, everywhere she goes, she seems to relish in bringing 'drama' with her. I think it is a good call to stop the drama.

  22. Here, Here!!!!
    Fantastic posting!

    I find it amusing the 'good doc' has been here. She makes me crazy.

  23. Good on you! You've become her latest obsession!

  24. rixa.

    this might sound dumb, but now i am confused about my cycle...if my "cycle is 27 days" according to what you wrote when the heck do I ovulate?

    Shouldn't my ovulation period start on say..this month, this coming sunday?

    again, i know that i might sound dumb...but i have always thought that i do ovulate 14 days or around there after my "period"...

  25. There's no hard and fast rule about when ovulation occurs, but the period between ovulation and the next period is usually more consistent (called the luteal phase), than the number of days from the start of your period to when you next ovulate. In other words, the luteal phase of women's cycles is usually more consistent (12-14 days) than the 1st half, which can vary widely from woman to woman.

    One way to find out when you ovulate is to chart your fertility signs. My favorite book for this is "Taking Charge of Your Fertility." Based on a few signs (mainly basal body temperature & cervical mucous) you can know when you ovulate.

    For example, my cycle is 26-27 days usually, but I have a short luteal phase of usually 8-10 days. So I ovulate later in my cycle than I would have predicted.

  26. oh that silly "doctor". really. the evidence outweighs the fear in this instance. how arrogant the medical model finds themselves, believing that the woman's body is always out to get that little baby. they must come in and save!

    I am so glad you chose to remove her posts. It's not about civil dialogue - it's about her need to be right all the time. Which is so "doctor" like. :)

    I should post the link to my postdates informed choice agreement. Because the evidence is solid...I appreciate research and true studies. I don't like anecdotes and "beliefs" unless they're rooted in research and/or the basic physiology of birth.

  27. oh and I cannot figure out why your blog isn't in my blogroll. darnittoheck! (yeah, i would say worse, but only on my blog)

    sorry about that!

  28. A friend gave ma a link to this one....BRAVO!!!!! I am glad I am not the only insane woman in the world that hates the dreaded "Due Date" phrase!!! I have carried right at 43 weeks with two of was a medical induction at 37 weeks, and the fourth was induced at 40 weeks because of my husband being deployed...that one was hard for me, but he wanted to be there so badly and they would not have sent him back from Iraq if I didn't have the baby while he was home on the dates they chose (happened to be around my "due date"). Anyway, after much much prayer we decided to go ahead and induce. That meant so much to him to be able to be there.

    Anyway I am getting off track first and third babies I went 43 weeks with each. Constantly changing dates because of ultrasounds, dr's always freaking out, etc. I have medical issues that make it impossible for me to use a midwife, but if I could I definitely would!!!

    I know someone brought up the mom factor...I do not take after my moms side medically, I take after my dad's. My mom never went late (early on all three, one by several months)...she had very quick labors also. I was in labor more with my first before we even went to the hospital then my mom was total with all three combined.
    Anyway, just wanted to throw that in there...I guess I am one of those that totally likes to throw a wrench into statistical

  29. Hi, this is my first time pregnant and i am anxious like all first time moms. I was given an EDD as 30/03/08 but since in my third trimeter i did an ultrasound scan which put the EDD to 14/04/08. This leaves me confused and a bit anxious but doctor says all is well, is there anything i should be worried about?

  30. I have extremely, extremely irregular cycles, so all of my due dates have been calculated by early ultrasound. My first baby came the day after her EDD, but my water actually broke the morning of her due date. My second was born a week and a half before his EDD, three days after a stretch and sweep. My third was born on his due date. Currently I'm 7 or 8 weeks along with number four (I remember ovulating but for the life of me I can't WHEN it was, haha!), and it'll be interesting to see how close to the EDD this one comes. I'm not trying to really say anything important with all of this, other than, that for me, the early u/s tends to be pretty accurate.

    My mom, on the other hand, had 8, and each of us was about two weeks late, but she also had really irregular periods and I bet her dates were set by the 28 day rule.


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