The Due Date Margin of Error

2008 US gestation distributionWhether you want to schedule a last fling before the baby comes, or this is your nth kid and you need someone to babysit the other n – 1, knowing the margin of error on your due date would make planning a lot easier.

This knowledge can also give dads credibility in certain parenting circles and in baby-related decision making, which may help boost their confidence and moral.

Below are some US margins of error and a few other handy statistics that will quickly prove you are an engaged and caring father. Warning: also included below are some intimidating pregnancy words (e.g., gestation, menstrual, conception). Apologies for any anxiety they may cause.

Information and Sources

It’s hard for a dad to get good information when something like 98.6% of statistics are made up on the spot. Online are a variety of due date confidence intervals, from 12 days, to 18 days, to 4 weeks; so there’s a large margin of error on the margins of error. Also, someone said that 60% of people don’t credit their sources.

Tired of the confusion and in need of a moral boost, as we’re approaching n = 4, I decided to go to a pretty reliable source – the CDC birth data warehouse. The stats below are based on the latest CDC data set, a 3.2 GB file containing natality information on the 4.26 million births registered in the US in 2008.

Some Background

First, some background info that I recently learned. At the start of the pregnancy you have a due date – when the baby is most likely to be born, based on when the doctor estimates the baby was conceived. Once the baby arrives, you can translate this date into an estimate of gestation, the number of days or weeks prego, by counting back to the estimated date of conception.

Stay with me.

The standard initial due date estimate is 280 days from the last menstrual period (LMP; ask your wife if you’re confused), or about 266 days from conception. This estimate is sometimes adjusted based on ultrasounds and such, but it’s typically close to 40/38 weeks.

So – we’re interested in the average gestation for women in the US and the variability around that average. The average, 38 weeks, is what they tell us at the early prenatal visits. The variability is key, as it will tell us the likelihood of the 38 for a randomly selected case, i.e, for our next kid.

The Stats

The plot above shows the adjusted gestation distribution for single births, in percentages. And the table below contains the mean, standard deviation, skewness, kurtosis, and count, for the adjusted and LMP gestations in 2008. I removed extreme outliers, flagged records, and missing data, which brought the counts down to under 4 million. Note that the adjusted estimates are much more accurate.

Some Gestation Stats
Estimate Mean SD Skew Kurtosis N
Adjusted 38.59 1.94 -3.21 22.61 3665402
LMP 38.73 2.37 -1.77 12.82 3672562

The take home message: due date margins are like the big stretchy waist bands on maternity pants. Only 75% of expectant moms have their baby within 10 days of the standard due date; 85% fall between weeks 37 and 40, making a huge 28-day margin. Plan accordingly.

By the way: 4.26 million births? That’s 8 per minute!

Canoe Camping on the St Croix

Sunrise on the St Croix

We’ve had three successful canoe trips with the kids this summer – two with very reasonable crying/whining to fun ratios (Minneapolis chain, Phelan chain), and one with nearly as much crying and frustration as fun (we capsized – more to come in a later post). On average, I think we were all ready for our first overnight river voyage.

There are plenty of rivers to choose from in the land of lakes, but not many that met our requirements of being kid-friendly and close to the cities, with free camping. We charted a course on the St Croix, which creates most of the border between MN and the land of cheese heads to our east. The St Croix is contained within a national scenic river-way, so the campsites are easy to find and well maintained, unlike those on the Minnesota river (see here).

We convinced a friend to come with his son, which put the crew at two dads manning the oars, and three kids crammed in the middle of the boat with all the gear. Starting at Interstate State Park, near Taylors Falls, MN, we made it to the Eagles Nest group camp (pics below) just before dusk. The site was excellent, and we would have enjoyed an evening by the campfire, but no piece of foliage was dry enough to burn, not even the dead leaves and pine needles (not the first time it has happened – see here).

Add the lack of campfire to a thick fog of mosquitoes and we had a campsite that was really only worth sleeping in. But, the rest of the trip went well, with much more fun than crying. The kids enjoyed it, and that’s pretty much all that matters, right?

I’d do it again just to see the sunrise over the river – it gave me goosebumps, at least, I thought it did. They turned out to be mosquito bites.

Canoeing the St Croix

Eagles nest campground on the St Croix

Eagles nest campground on the St Croix