The Length of Pregnancy


The length of pregnancy is a fascinating topic to me because, if you've ever been pregnant, 40 weeks seems like a really long time an...

The length of pregnancy is a fascinating topic to me because, if you've ever been pregnant, 40 weeks seems like a really long time and calling it a "guess date" is a little frustrating when you might need to plan things around that due date (ie. if family needs to take time off to visit and needs to give advance notice at work). I've heard a lot of women opting for inductions and c-sections because they want to know when their baby will be born. I think medical interventions should be saved for the times they are genuinely needed, but I GET where they're coming from. If I go into labor for my VBAC (vaginal birth after c-section) and end up needing a c-section, I have no guarantees that we'll have family available or able to take off work at that time to come help out. My husband gets paternity leave (yay!) so it should be fine, but if he didn't we would probably be in trouble because for a few weeks after a c-section you're pretty much out of commission. Not great with a newborn AND a toddler. Having family around to help is nice and with so many people like us who have family living long distance, it's hard to figure out what to do when you don't KNOW your due date or what kind of help you'll need.

Today I saw this really interesting article on the Length of Pregnancy; it's not good news for us mamas who would reallllllly like to KNOW, but it's interesting anyways. While they used to believe that the up to 5 weeks variation in pregnancy length was due to miscalculations of ovulation, this study actually tested women to determine exactly when they ovulated. Neat! Not sure why no one thought of this study before though...?

Main Points from Article

  • "A woman's due date is currently calculated as 280 days after the onset of her last menstruation and is only ever considered a best guess. Only four per cent of women deliver on their due date and only 70 per cent within 10 days of it."
  •  "...average time from ovulation to birth was 268 days - 38 weeks and two days"
  • "The length of a pregnancy could be affected by a number of factors. Older women tended to deliver later, while women who had themselves been heavier at birth also had longer pregnancies."

    Link to the Original Article: Length of pregnancy can vary by up to five weeks, scientists discover
Do keep in mind that a major flaw to this study was the small sample size that they used. 

Other Considerations

From that article, we might calculate our approximate ovulation date (I use an application on my iPhone to track my menstrual cycle and it actually gives me my approximate ovulation dates, but I also know people who use ovulation kits which is more accurate) as being 268 days from our ovulation date. According to an article, "ovulation typically occurs around 14 or 15 days from the first day of the female's last menstruation cycle." My phone application actually didn't calculate my ovulation using that standard; in fact, I'm really not sure WHAT standard it was using. I think it averages your lengths of cycle (mine, of course, had been somewhat inconsistent due to breast feeding) and bases your ovulation on that. 

We could also calculate it with the general standard, 280 days after our last menstrual period. Here is a link to a calculator from BabyCenter to help you figure out that date.

The third method is to use a dating ultrasound. Here is a link to an article about ultrasounds during pregnancy that brings up the accuracy of the dating ultrasound. This article states, "...when the margin of error is reported as a percent of the total, it is calculated against the number of days gestation verses the number of weeks. In other words, an 8% margin of error at 8 weeks' (or 56 days') gestation calculates to ± 5 days; at 18 weeks' (126 days') gestation, this represents a range of ± 10.08 days; lastly, at 28 weeks' (196 days') gestation, the margin of error continues to widen to ± 15.68 days. Regardless of these inconsistencies in reporting, most concur that an ultrasound performed before 24 weeks' gestation establishes a more accurate EDD than relying solely on the LMP." As I had a dating ultrasound around 7-8 weeks, my "guess date" per the dating ultrasound should be +/- 5 days. I'd be interested to know how much variation there is in the EDD being based on the dating ultrasound when it's done transvaginally vs. the typical external ultrasound; my understanding is that the transvaginal ultrasound is more accurate, but that's just what I've heard from other people and not what I've actually seen reported. I need unlimited access to clinical studies because there's a lot of questionable information on the internet! 

My Due Date

I've been just saying that K will be here in early November as this seems to be the safest thing to say. But just for fun, I did some math! Haha. If you didn't read my earlier post about my issues with doctors and their tests, here it is. To summarize, my old OBGYN (who I fired not for making the mistake, but for their really terrible way of letting me know then lack of communication when I had questions) told me I wasn't pregnant at the beginning of the pregnancy. Awesome stuff. I have more information from this pregnancy than I do about G's pregnancy as a result.

G's Due Dates Based on These Standards
I wanted to look at my previous pregnancy with G, who was born on September 30, 2011, just as a comparison to see what was most accurate for ME during THAT pregnancy. 
  • Based on 268 day average: EDD would have been September 29, 2011 (based on ovulating 15 days after LMP)
  • Based on the 280 day LMP estimate, his EDD was September 26, 2011. 
  • With G I didn't have a dating ultrasound until 12 weeks, I believe, which may not be as accurate as an 8 week ultrasound (it was also external), but according to that estimate his EDD was September 23, 2011. 
G measured big the whole pregnancy ("ahead" even at the 12 week ultrasound) and still came according to the 268 day average that the study came up with. 

Estimating K's Due Date
  • Based on 268 day average: October 13, 2013 (or if you use my phone's ovulation estimates, October 20 or so). 
  • Based on 280 days standard: October 10, 2013
  • Based on my 7-8 week ultrasound: October 31, 2013 (this is my official due date because it's the most accurate means my midwives had of determining EDD that actually has better). 
Kaden is measuring "big" according to the October 31 due date (3 weeks ahead at our 24 wk appointment). Based on HCG levels taken on March 9, 2013, I was 7-8 weeks pregnant at that time which suggests the baby was "born" (ovulation occurred?) on January 12-19. Per that info, we could estimate that according to the 268 day average, I'd be due October 7-14 and based on 280 day average, I'd be due on October 19-26. 

My Guess
There's a trend in our mom's group where everyone seems to be having second babies on holidays (excluding inductions). We've had a Father's Day and Mother's Day baby so far! Per that trend, I'm having a Halloween baby for sure. Haha. 

Using previous pregnancy experience, I'm going to assume the 280 day standard is most accurate. Of course, there's a lot of variables we don't know there. And if we assume that, we have to question why K was measuring so small on the ultrasound, considering that I have measured large with him on every other measurement AND also did with G. Of course, as I found out with my negative pregnancy test from the OBGYN early in pregnancy, medical professionals make mistakes so the October 31 estimate could be off due to individual error. 
  • My OFFICIAL guess for due date is October 15, 2013. Why? Because the 268 day average was most accurate last time and one day short, PLUS that's my birthday. So it sort of qualifies for the "holiday" status too. I know I added an extra day just to make it on my birthday, but yeah. That's my guess!
  • Daddy B's OFFICIAL guess for due date is October 20, 2013. 
Pstt... he has to make me dinner if I'm right. If he's right, he still has to make me dinner. Haha. 

Obviously this all just means that the baby will most likely be born in mid to late October. At my last midwife checkup, she sounded like she'd REALLY like the baby to be early (ie. not wait until Halloween) due to G's large birth size, but our induction options are limited due to the prior c-section so it's really just a waiting game. We won't be stressing over our date, but guessing sure is fun!

What was the most accurate math for your child's birth? Did you have any big discrepancies for due dates for you? Anyone want to make guesses on our due date? The grand prize winner gets: self satisfaction. 

Note: Please excuse any potential mathematical errors that may have occurred in this post... I can't vouch for my math skills. But I'm pretty positive I've got the numbers right. Ha. 


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DIY Danielle: The Length of Pregnancy
The Length of Pregnancy
DIY Danielle
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