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!