What Is A Due Date, Really? |

What Is A Due Date, Really?

admin September 6, 2012

What Is A Due Date, Really?

When a woman finds out she’s pregnant, one of the first things she wants to know is her is due date.  There are formulas for figuring it out manually, but most use a little “wheel” (like the doctors) or an online “quiz” that will tell you your baby’s due date.  Many first-time moms circle this due date on the calendar, as well as the date three weeks prior, when they reach 37 weeks or “full term.”  These are major milestones for them!

However, due dates aren’t all they’re cracked up to be, for several reasons.  And since due dates are (unfortunately) used to make serious decisions about things like labor inductions and scheduled c-sections (which we’ll talk about next week), it’s important to understand them.

What’s in a Due Date?

A due date is typically calculated from your last menstrual period.  It is based on the idea that pregnancy lasts approximately 280 days from this point, or about 40 weeks or 9 months.  An OB noticed that many women seemed to have their babies around this time, and came up with a formula for finding out approximately when to expect babies to be born.  As medical science advanced, they noticed that babies born after around 37 weeks were typically fully developed and did not have any major lung or other body system issues.  This became considered “full term.”

When you find out you are pregnant, your doctor or midwife will typically use a little wheel based on the averages above and your last period to come up with an approximate due date for you.  But don’t be so quick to circle it on the calendar!  A lot of factors can throw off your due date, sometimes by weeks.

Why Aren’t Due Dates (Necessarily) Accurate?

The idea that a due date can be derived from your last period assumes that you (and all women) have regular 28-day cycles, and that ovulation occurs around day 14.  This, as we know, is not true for all women.  Since the due date is calculated from your period, which is at least two weeks prior to your conception date, an irregular cycle can really throw a wrench in the mix.

These are some factors that can throw off your due date:

Irregular Cycle

Ovulation typically occurs about 14 days before the start of your next period.  If you have a 28-day cycle, that’s also roughly 14 days after your last one.  But if you have 35-day cycles, or 60-day cycles (which some women do), or 18-day cycles — any variation — your ovulation won’t have occurred “right in the middle.”  This means your baby could be due a week or two (or more) earlier or later than your calculated due date!  If you have irregular cycles, it is very important to let your care provider know so they can attempt to adjust for this.

Conception Date

You may know your baby’s conception date.  You may not.  It may or may not line up with when the calendar says you “should” have ovulated.  This usually will only throw things off by a day or two, but it’s still important.  If you’re familiar with your body and know what ovulation ‘feels’ like, or have a BBT chart, share your conception date with your doctor for more accurate dating.

Rate of Development

All babies develop differently, even before they’re born.  Some babies may only need 38 weeks.  Some need 46!  It is impossible to tell how long your baby will really need to be inside before being developed enough to be born.  Due dates are mere averages and guesses; they are not (and cannot) be exact.


Many providers will send women to get ultrasounds in their first trimesters to “date” the pregnancy.  While this is more accurate in the first 12 weeks than beyond (because the development of the baby from a ball of cells to a tiny “baby” appearance is more pronounced and obvious), it is still not a guarantee.  This date can still be off by a few days or a couple of weeks.  An ultrasound dated my first being due a few days later than she actually was (thankfully we didn’t induce, but she wasn’t as “early” as they thought — she was actually 4 days early.  I knew my conception date for certain and they used the ultrasound over my knowledge).

Why Does This Matter?

Unfortunately, since most modern OBs believe strongly in “the due date,” a lot of decisions can be made or strongly suggested about your care based on the due date.  When I went in for my 39-week appointment with my first baby, everything was looking great, baby and I were healthy, but the doctor said, “When you come back next week, we’ll discuss induction.”  Just because I had reached my due date!  I went home crushed.  (I had the baby 3 days later on my own.)

Recommending induction, or “allowing” induction (if a woman is ‘tired of being pregnant’) often happens based on the due date.  Some providers will schedule inductions any time past 37 weeks, and certainly past 39.  They rarely discuss the risks of this, either.  Most providers want to discuss induction by 40 – 41 weeks, and very few will “let” their clients go past 41 weeks.

The average first pregnancy for a healthy woman who is not induced is 41 weeks.  Average.  There will be women who naturally deliver at 39, and women who naturally deliver at 43.  And that’s all assuming that their dates are really correct in the first place!

When a baby is ready to be born is so subjective.  How many weeks the baby is does not tell you how big the baby is, how developed the lungs are, how developed the sucking reflex is, etc.  It does not tell you if the baby is actually ready to live on the outside.

Informed Consent With Due Dates

More and more doctors and midwives are starting to do away with a “due date model.”  Instead, they’re looking at a “due week.”  That is, for example, “We think the baby will be born around the second week of March.”  (Which is my “due time” with baby #4.)  This is a much better model because it allows for flexibility, rather than pinning one’s hopes and decisions on just one date.

Even so, informed consent is important.  Unless there is a medical reason (you have high blood pressure, baby is in distress, you have developed pre-eclampsia, your water breaks, etc.), there is no need to suggest interventions up to and including inductions and c-sections based on how long you have been pregnant.

Ask your doctor for evidence-based, individual care.  You do not have to consent to any procedure based on the fact that it is the office or doctor’s policy to do something after a woman reaches a certain point in her pregnancy.  Be aware of that, and speak up.  “As long as my baby and I are doing well, I prefer not to be induced, even if I go post-date.”

Next week we will talk specifically about induction — risks, benefits, situations where you may consider it, and so on.

What do you think about due dates?  Do you circle them on your calendar or do you take them with a grain of salt?

This is the writings of:

  1. I actually told everyone my DD was the day I started my 42nd week. I can’t stand the nagging when the DD gets close…


  2. My baby was born 16 days past his due date. I don’t know anyone else who has gone that “late” because most doctors would have induced long before that point. I was on the brink of having to be referred from my midwife to an OB when I went into labor thanks to acupuncture. If I had been referred to an OB I would have refused induction because all indications were that baby and I were both healthy and thriving. I was having a wonderful pregnancy and I wasn’t “ready to be done” as so many women are at that point. I was content to let him come when he was ready. I like to think he was born right on time! 🙂


  3. I always take them with a grain of salt, though I do ‘circle the date’ on the calendar. With my first I fully expected to go a week or two past the assigned date. But was completely surprised when she came 10 days early. The funny thing was that the placenta had already started to age so really she came right on time. I say our bodies know best & just relax. It’s the best thing for both baby and Momma!


  4. I generally give a “due month” – easier to do when you’re due early-mid month. :p That said, I’m big into ovulation-based due dates. My last baby would’ve been due a full month before his REAL due date if I went by last menstrual period. Which would’ve meant he came at 43+1 instead of 39+1. :p


  5. The only people who know our EDD is me,my husband and our midwife. We tell everyone else “Oh the end of September/beginning/middle of October.” They HATE not getting a date, most will keep asking. lol so then we change to well its really whenever the baby is ready. We are doing so many “weird” things that people have just stopped asking us any questions {sad}


  6. If I have another baby I will definitely add 2 weeks to my “guess date” as #2 came at 42 weeks and #3 came at 41.5 weeks. Way too much attention and concern from others those last weeks.


  7. Such an informative post. I guess the best way to tell someone when you are due is by telling them the month. Babies are unpredictable so it could be 2 weeks before a week after, its very rare for a baby to come out on its due date.


  8. […]  What is a Due Date, Really? at Modern Alternative Pregnancy “…However, due dates aren


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Hi, I’m Kate.  I love medical freedom, sharing natural remedies, developing real food recipes, and gentle parenting. My goal is to teach you how to live your life free from Big Pharma, Big Food, and Big Government by learning about herbs, cooking, and sustainable practices.

I’m the author of Natural Remedies for Kids and the owner and lead herbalist at EarthleyI hope you’ll join me on the journey to a free and healthy life!

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