Why Pregnancy Due Dates Are Inaccurate

My first pregnancy was 44 weeks +1 day long if you use our 'standard' measure of LMP to figure the due date. I should have expected as much. I was born at 44 weeks, as were 3 of my siblings, and 8 of my grandmother's babies. Normal gestation length tends to run the same in the maternal line of families. They are "hereditary" in this manner. And some babies just need a little extra grow-time before their lungs are fully complete and ready for the outside world.

By the time 42 weeks rolled around everyone was asking me when "they" were going to induce me. But "they" were not going to do anything to me! No one stays pregnant forever, and babies know when it is their perfect time to be born.

This did not stop strangers from screaming across parking lots, "Are you having twins?!" and "You are HUGE!!" haha... Thanks! And no, I have just one little person busily growing inside, thankyouverymuch.

In any event, it would do us much good (and serve our babies in providing them strong, healthy starts in life) to let them trigger labor in their own good time. The lungs are the last vital organ to finish developing before baby's hormones kick-start mom's hormones in the triggering of labor and birth. We would see a great reduction in the rate of SIDS (and other maladies) if we let babies' lungs complete this vital process, rather than jumping the gun in our eager anticipation to pull baby from within in our (frequently misjudged) timing.

Our epidemic rate of induction, use of pitocin, and sky-rocketing cesarean sections are directly related to our lack of trust in birth and babies and women's bodies. Our children are paying the consequences. We must realize that our guess dates are simply that -- guesses -- they are not realistic time limits under which human babies must comply in their inutero development. Leave them be. Let them come when they are ready. No induction is normal.

The following by Lucy Myers
Posted with permission. 
Read more from Myers here.  

Pregnant women and their care providers, as well as society in general, put a lot of stock in due dates. But in reality, a due date is only an estimated date of delivery (EDD). Mothers-to-be, midwives, doctors, families, and others would do well to remember the following reasons why due dates are often inaccurate.

1. Length of the menstrual cycle. Every woman's due date is calculated with the assumption that she has a 28-day menstrual cycle. In addition, it is assumed she ovulated 14 days before her last menstrual cycle began. The due date, then, is 40 weeks from her last menstrual period.

This method of calculating due dates is extremely out-dated, presumptuous, and flawed. First of all, not all women have regular menstrual cycles. Secondly, one woman may have a 28-day cycle, while a second woman's cycle is 35 days, and a third's is 45 days. Healthy women have menstrual cycles of various lengths, and the length can even change throughout a woman's life or from one pregnancy to the next. As a result, due dates are often inaccurate.

2. Length of pregnancy. Believe it or not, not all pregnancies are 40 weeks, nor do they have to be. Technically, based on the 28-day menstrual cycle, a pregnancy whose due date is calculated using the above method would only be 38 weeks gestation. This is because, if ovulation happens on day 14, there were 2 weeks between the last period and ovulation. So, saying the pregnancy lasts 40 weeks is wrong, even with this faulty due date calculation.

However, not all pregnancies are 38 full weeks, either. This is simply because women are all different, and so are their babies. A healthy woman can have, say, 3 healthy babies, all born at different gestational ages. There is really no concrete way to determine how long a pregnancy should or will be. But one woman was pregnant for an entire year--52 weeks! The mother and baby were completely healthy and never in danger. A year-long pregnancy was just what that particular mother and baby needed.

3. Ultrasounds can be off. No matter how good ultrasound technology becomes, measurements of fetal weight and size can be off. So can the due dates ultrasounds provide. Dating a pregnancy by using ultrasound is only accurate within the first few weeks of pregnancy. But even then, the due date can be anywhere from 2 to 4 weeks earlier, or later.

4. Average gestation. There is no magic 40-week mark, as care providers would have mothers believe, and even as they themselves believe. Instead, the average length of pregnancy is actually 41 to 42 weeks. As a result, due dates cause needless worry in mothers and care providers. However, things like induction, if considered at all, shouldn't even be remotely thought of until at least 42.5 weeks gestation.

There is much to be improved, especially within the United States, in obstetrical care. And it may never be at the point where mothers and care providers have adult, enlightened, fact-based conversations about care during pregnancy. But both sides need to be truly informed. And one step in the right direction is realizing the flawed nature of due dates. If there is no logical, rational, medical reason to think mother or baby is in danger, the baby should be left alone as long as he or she wishes. That is, labor should be allowed to begin spontaneously.

This is an unpopular view, because in today's society, women want their babies out by elective cesareans, because they're uncomfortable. Big deal! Would you rather have a baby born at "40 weeks" who was actually 4 weeks early, staying in the NICU, having trouble breathing, etc, just because you were uncomfortable? Or would you rather wait 4 weeks, be extremely uncomfortable and angry, but end up with a healthy baby to ease your mind and bring joy to your heart?

See Also:

The Lie of the Estimated Due Date: Why Your Your Due Date Is Not What You Think It Is

No Induction is Normal

Fetal Lung Protein Release Triggers Labor to Begin


  1. I went "overdue" as well. My DD was born at 42 weeks, 4 days, according to my LMP. However, I was charting and ovulated early that month and she was actually born on 43 weeks on the dot. Some babies just take longer and that's ok. :)

  2. The more of your posts I read the more determined I become about my next pregnancy & delivery! The midwives were desperate to induce me with my now 10 month old daughter- luckily I refused! But they were putting ALOT of pressure on me, mainly telling me that I was risking the health of my baby etc, etc. In the end she was born 41 weeks and 3 days but in the UK they push for induction asap. This post just shows they don't even know themselves when the baby is 'due'!

  3. Great info. I also do not believe highly in induction and elective c-sections because of the fact that no one knows the supposed "due" date except God! OB's and some midwives use scare tactics to make patients believe their unborn baby is really in danger just because they have so much trust in a date. And as mentioned, babies lungs need that full-term time to properly develop so baby is strong and comes out when he/she wants =)

  4. I went 34.2 with my DD. Thankfully my HB midwives were fine with that and just requested that I have a non stress test once at 42.5.

    The hospital freaked out when I went for the test and tried to admit me for an induction on the spot. I happily left with dropped jaws following me out the door.

    DD knew when she was ready...

  5. this is a topic that really makes me very upset! why arent more people thinking? its like, everyone is just concerned with themselves, not their baby.

    i try to tell people all the time; its funny that pitocin use, c-sections and SIDS are rising all at the same time, and they look at me like im nuts. i mean, how hard is it to understand that a baby who mysteriously stops breathing could have something to do with lungs that are not properly developed? of course, we get to the fine line there where we seem to be "blaming" a mother for electing that route and "causing" the death. but really, i dont know any mom who had a c-section who was aware of or understood the risks, and the process enough to really make the decision anyway.

    i dont know. its just insanity. why cant we just UN-medicate labor, EDUCATE moms with real, honest information about pregnancy, labor and delivery, and give them the tools they need to make their decisions WITHOUT the seeming pressure to induce, medicate, circ, formula feed, on and on and on.

  6. The number one cause of premature births is induction. Why, why are we in such a hurry to rush nature. Sigh.

    Pregnancy is temporary, birth is momentary, and the memory life lasting.

  7. And then you can be me.. My mother has babies at 40-44 weeks 7-9 1/2 lb. I deliver 36-39 weeks. Only twice out of 7 have a made it past 38 weeks. All my babies are healthy and were good sized for there gestation over 6-7 lbs. I homebirth also. It is so variable. I tend to not follow my mothers pattern for pregnancy, birth at all

  8. contrary to what one post on here says....in the UK induction is normally recommended at 42 weeks for post-dates in otherwise normal pregnancies. Only if there is another (fetal or maternal)reason is it recommended before then.

  9. I agree generally with your post with the caveat that sometimes babies do come earlier than is for their own good. My four kids were born at 38w+5 (9#6, 24.5"), 38w+5 (7#13, 23.5"), 40w+2 (10#, 23.5"), and 36w+6 (8#6, 21.5"). All labors were spontaneous. The first 3 were born at home, uneventfully; #4 was born at a freestanding birth center and due to breathing problems (persistence of fetal circulation and hyaline membrane disease) required nearly a month in the NICU before coming home. As you can see, I grow them big and had #4 stayed in until his due date, I'm convinced he'd have been 11#. Which is OK, whereas the fact that he came early wasn't so good for him. So why did he come early, when his lungs weren't ready? We will never know. He's healthy and happy now though!

  10. I have a 40 week first-time Mom that I'm staying with (out-of-state doulary) and she and I are both fascinated by the 52 week pregnancy. Can we get a citation on that? When and where was it?

    1. I've wanted to know the same thing since reading this article! I believe it absolutely needs a citation!! I am fascinated as well. The best thing I have been able to find so far is this, and even this leaves me wanting more!

  11. Here's one TIME report on a 54 week pregnancy http://www.time.com/time/magazine/article/0,9171,797153,00.html

  12. You can get a weekly test done to make sure the placenta is functioning normally past your guess date. If I didn't go into labor on my own each time by around that date, I would have waited till I did, but just check in to see that the placenta was still functioning at full performance.

  13. I practice in a hospital where we recommend induction of labour around (but not before) 42 weeks. Inducing labour before this time can lead to problems in labour. The cervix may not be ready to dilate and earlier induction does carry an increased risk of caesarean section for slow labour or 'failed' induction. After 42 weeks it is less likely that induction of labour will alter the final mode of delivery (although it will, of course, impact birth and normal hormone rushes).

    There are a lot of studies which have looked into the ways we can monitor women after 41 weeks time to try and pick out those that really do warrant induction, and leaving the majority to labour naturally. This has included surveillance techniques such as kick charts, fetal heart monitoring, ultrasound scans to check for fluid volume, and doppler assessment of blood flow in the cord. None of these is perfect, but all are better than premature induction when a mother would otherwise gestate just fine for her needed amount of time.

    The chances of spontaneously going into labour after someone's "due date" are very good, indicating no need for induction. When a woman reaches 40 weeks gestation, 65% of the time, she will spontaneously go into labour within the next 7 days. For those who reach 42 weeks gestation, 60% will naturally enter labour within the next 3 days. The odds are in favor of trusting birth and trusting babies to trigger labour in their own timing.

  14. What frustrates me is that in my state homebirth midwives are not allowed to attend women past 42 weeks. So there is pressure to have babies before 42 weeks even with homebirths. I know of midwives doing "natural" induction techniques starting at 37 weeks in order to prevent moms from going "over due" and facing a hospital birth. How do you fix this problem?!

  15. For me, the opposite is true. I have 4 children, 1 girl and 3 boys. My sons have all been born between 35 and 37 weeks. Their weights were between 5 lbs. 1 oz. and 5 lbs. 7 oz. None spent anytime in the NICU & breastfed almost immediately after they were born. My daughter was born at 38 weeks and weighed 6 lbs. 12 oz. She was just fine also.
    What bothers me is seeing so much info on staying pregnant(which I agree with)regardless of healthcare providers advice and seeing almost no info on how babies born as "early" as mine can still be just fine w/o all the medical measures they want to apply. Granted I do things my way no matter what "they" say but some Moms need more info in the other direction. What do I do if my child is born "early"?
    I had rounds w/ the hospital docs. I was told you can't room in, the baby needs to stay in the nursery. Why? Is something wrong. Nothing that we can tell, he was just so early. REALLY? That is all you based it on. Apgars were fine and breastfeeding was implemented even before the placenta was passed and you still want to keep my baby away from me.
    Moms who have babies "early" need to know how to handle things.
    I have to post anon but I'm Angela Cope on FB and I read your posts all the time.

  16. Wholly Mama ~ if you know that long gestation runs in your family, you simply add on 2 weeks when first reporting your last menstrual cycle at your first appointment. This will 'buy some time' with homebirth midwives and keep them (legally) in the clear. My own midwife (knowing all the women in my family birthed at 44 weeks) suggested this "off the books" and when I filled out my intake paperwork, this is exactly what I did - added weeks on (saying my LMP was 2 weeks later than it actually had been).

    Angela ~ You bring up a very interesting perspective that we should certainly address more often. Some women have naturally short gestation lengths and their babies are also just fine - perfect and healthy and triggered labor in their own time. They, too, do not need all the unnecessary intervention and fuss surrounding them. They just need their momma, her warmth, her body, her milk. Hope we can find more research surrounding this aspect of birth and babies too!

  17. my babies have all been around the 40 week mark so don't really know much about going over due 1st 40+1, 2nd 39+6, 3rd 38+4 by scan (39+4 by lmp) 4th 40+6 by scan (39+5 by lmp) all been totally fine apgars of 9 1 min then 10 at 5 (think 2nd was 5 mins but can't remember lol ) anyway my mw knows that i won't even accept a stretch and sweep till at least 42 weeks and is fine with this as long as i accept nst's which i'd fine with , my edd by ovulation this time is 10th of may but by dating scan is the 6th the mw said she has to use the dating scan though i think that is silly :/ all my births so far have been homebirths but this is going to be hospital because of gbs , i don't want it to be but i don't feel i have any choice as just by asking the safety of antibiotics i get jumped on and told my baby will die without them :( sorry for going off topic there

  18. Becky ~ I would re-do the GBS test. They can be positive one day, and negative the next. Just because you tested positive one day, does not mean you will be today, nor does it mean you will be on your birthing day. And equally, there are some who test negative, and then are positive when birth comes around (and they never know). Using Hibiclens is an option some women choose for homebirth or to avoid testing positive if they plan to have a hospital birth. Here is more info:





  19. Great Article! Thank-you.

    I went into labor at 38 weeks and my cycle was 28 days long. I wish I had read this article first and I wouldn't have been so surprised (and unprepared) for the 'early' home birth of my daughter.

  20. Both of my children were "late." A- was 9 days "late." L- was 8 days "late." Maybe baby number 3 will be exactly 7 days "late." ;)

  21. Yeah...mine was 27 weeks... ;-) talk about messing up the average! I wish more OB's would spend time impressing this upon mothers. I talk to so many who are convinced at 32, 35 wks that they will go early and cannot fathom hitting, much less passing their due dates. When it happens, it is hard to deal with. I have never reached my edd...but I never rule it out as a possibility, not even the eighth time around. I have great respect and awe for those who have continued on to 41, 42, even 43+ weeks.

  22. Both of my daughters went past their due dates... and I've been waiting on them ever since! :)

  23. "Anonymous" suggests that in the UK induuction is only recommended at 42 weeks. Hogwash! Every Mum I know had an induction "offered" at term + 10 at the very latest. Usually along with the caveat that "babies die at 42 weeks". B*stards.

  24. I agree and carried my babies to 41.6 and 41.4 weeks before having naturally occuring labor. As a doula being past due is a major concern of many of my clients. Most really fall for the placenta fear - they are told it no longer works optimally after 40 weeks. I have never been abe to find a good resource for calming those fears. The other is the fear of an in utero bowel movement, again planted by doctors with threats of mother/baby seperation, antibiotic use, even death if a mother "allows" it to occur.

  25. I was scheduled to be induced 10 days past my due date, when I went in I was starting to contract on my own but they were not ones that I was feeling but I was starting to dilate so they sent me home and if I didnt start feeling the contractions by the morning they wanted me to come back in to get checked. I didnt feel them and went back to the hospital the next morning like asked, they admitted me and started pitocin, just less than 12 hrs later I was holding my baby girl. She did poop while inside me still and my placenta "looked old and damaged" and they said that it was a good thing that I was induced.. I also had a low lying anterior placenta so not sure if that has anything to do with anything but yeah, I was glad to be induced rather than something possibly happening to my baby. She was also born on my dads birthday which made him very happy, he says "nothing can top that birthday present!" =]

  26. Ive had one spontaneous labour at 37wks - if you can call it that. I slipped in the shower, broke my pubic bone and my coccyx and went into labour from that. Hospital were assholes about it and I wish to this day I had stayed at home. I ended up strapped to the bed on continuous toco and it was just hell. Agony.

    Ive also had 2 inductions at similar times. One was admittedly social, as my hubs and I were posted to a new place, 2000km from home and family, and he was being deployed a week later. I had no other way to care for my 2yo son while in labour if I didnt have the baby then and there. My second though, we avoided as long as we could but in the end I was spilling autoantibodies and I was getting too sick. I had to trust their call. Realistically, I was lucky she was not miscarried, I was lucky to get to 30 weeks, and I was lucky to get to 36+5. But if I had the choice, they would all have stayed until they chose to come earthside.



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