How to Calculate Your Real Due Date

Adapting the due date to fit the mother.

Ok, let’s imagine you’re 39 weeks pregnant and approaching your due date. A few things are happening. You’re probably not sleeping so well anymore—getting up to pee 3+ times a night, hip and back pain from funky sleeping positions, and when you roll over, getting jabby abdominal cramps that feel like one big uterine charlie horse. You’re out of breath climbing the 3 stairs to your front door, you’re getting heartburn every time you eat a normal sized meal, your bones and muscles ache, your feet are swollen, the list goes on and on. It can feel like some serious hard work just surviving the day in one piece!

But here’s the thing: the toughest part about the last few weeks of pregnancy isn’t physical. The toughest part is the the psychological mind-game. When I was pregnant with my first baby, I told myself that I would never be the one to rush my baby out, that I wanted that baby to bake as long as possible and come whenever she was ready. I wouldn’t be the anxious, obsessive mother-to-be, binge-watching The Office on dvd trying to pass the time. Oh, how wrong I was. At the end of this pregnancy, I was doing all the things I now advise my doula clients against: I was taking my dog on speed walks around the neighborhood for hours at a time, eating all the things that I’d heard might kick labor into gear, calling my mother compulsively begging for details of when and how her labors started. I didn’t want to leave the house or make any plans in case I went into labor all at once, going from zero to transition in 20 minutes. (For those of you experienced parents, you know how hilarious that is!) I would go into my midwife’s office and anxiously shoot of a rapid fire list of questions she couldn’t possibly answer, like “when do you think I’ll go into labor?” and “ “exactly what will the first labor pains feel like?” I could not stand not knowing everything! I ended up going into labor 5 days after my estimated due date (EDD), but what an agonizing 5 days they were.

Yes, I was a complete mess those last weeks of pregnancy, wanting concrete answers to questions that honestly had none. No one could tell me when I would go into labor. No one could tell me what labor would feel like. It’s so completely different for each person each time. The mystery of birth is immense and unknowable, and yet when you’re 39 weeks pregnant you can feel it right on your doorstep, about to knock at any time.

I don’t want you to do what I did. So here’s a tip: expect to go late.

You might have heard this before, but this time I really want you to believe it. Think of your EDD. Now add 8 days. That’s your new due date (in your mind). We’ll call it your Realistic Due Date (RDD) for the purpose of this post. If your care provider told you that you’re due on November 12th, great, super, fantastic! But in your mind, I want you to think it’ll realistically be November 20th. If it’s January 29, I want you to think February 6th. (Oooh, I’m gettin’ tricky! It can actually be much more challenging if your due date is at the end of the month, with the extra 8 days spilling over to the next. You’ll have to practice extra to get that new month to stick! Tell your partner, your family, friends, and Facebook that you’re due on your RDD that way you won’t have all of those lovely, well-intentioned calls, texts, and posts on your EDD asking you if you’ve had the baby yet and to please, please, PLEASE let them know when you do.

If this feels dishonest, it’s not! There’s real research to back this up.

A 1990 Harvard study called “The Length of Uncomplicated Human Gestation,” referred to as the Mittendorf study, found that the average length of human gestation is 41 weeks and 1 day. That’s probably why only 4% of babies actually arrive on their due date! Many midwives and OBs know from practical experience that first timers, older moms and VBACs (Vaginal Birth After Cesarean) will generally go later than 40 weeks. And although I’m on call for all of my clients from 2 weeks before their EDD to 2 weeks after, I know I can usually expect a call in the middle of the night around 3-7 days post date. (Why not longer if 41 weeks + 1 day is the average? We’ll talk more about inductions and US birth culture in another post soon!)

One midwife, Carol Wood Nichols, actually developed a method of calculation for a more realistic due date by taking individual cycles into account. Here’s her formula, known as Nichol’s Rule.

  • First time moms with 28-day cycles: Last Menstrual Period (LMP) + 12 months – 2 months, 14 days = EDD
  • Second time moms or more with 28-day cycles: LMP + 12 months – 2 months, 18 days = EDD
  • Cycles longer than 28 days: EDD + (days in cycle – 28 days) = EDD
  • Cycles shorter than 28 days: EDD – (28 days – days in cycle) = EDD

I do apologize for the excessive amount of math you just had to do there. But isn’t this amazing? Instead of forcing each woman to conform to an arbitrary 40 week + 0 days rule, it puts her back at the center of the equation, saying “hey, look at the range of possibilities here—let’s adapt the due date to fit the mother!”

We already know that full term is 39 to 42 weeks—one recent study found that the length of pregnancy can naturally vary by up to 5 weeks. But having one specific date in mind can oftentimes feel like a deadline when you’re more than ready to meet your baby. So please, do yourself and your baby a favor. Add 8 days to your EDD now or find the Nichol’s Rule formula that works for your cycle and take a deep breath. Your body knows what it’s doing, your baby will come when s/he’s ready, and it just might give you some extra time to have a cup of tea and put those hard-working, swollen feet up for a bit!

Source Credits:

  • Davis, Elizabeth. Heart and Hands: A Midwife’s Guide to Pregnancy and Birth. 3rd ed. Ten Speed Press: Berkeley. 2012. Print.
  • Groskop, Viv. “The 40-week Pregnancy Myth Has Popped.” The Guardian. 8 August, 2013.
  • Nichols, Carol Wood. “Postdate Pregnancy, Part II: Clinical Implications.” Journal of Nurse Midwifery 30, no. 5 (1985): 259-68.
  • R. Mittendorf et al. “The Length of Uncomplicated human Gestation.” OB/GYN 75, no. 6. (1990): 929-32.
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