Thursday, August 22, 2013

Odds for betting on a birthday

Earlier today I tweeted about my maternity leave memo...
My maternity leave memo made finance staff laugh. What? Don't all moms incl a table w/ statistical likelihood of birth by week of gestation?
— Beth J (@DataGeekB)
The tweet generated as much commentary (and laughter) as the memo did, so here are some highlights of the data and answers to reader questions.

While plenty of practitioners will tell you that a "full term" birth is considered to be 40* weeks (280 days) of gestational age, the numbers simply don't bear that out. In recent years, both the median and the mode place a baby's most likely birth date at week 39 with a standard deviation of 7 to 9 days (depending on the source).
Source: CDC Births: Final Data for 2011, compiled by author
Note: Data shown here are for singleton births only, as multiple births have a higher likelihood of preterm delivery
I suspect the misconception about the most common length of pregnancy is a matter of old data.

In 1990 the median delivery date for a singleton** birth was in the 39th week, but the mode was 40. Perhaps most shockingly, in 1990 more than 10 percent of births occurred at 42 weeks or later!

Birth timing is not entirely predicted by biology. Medical interventions abound, as one in three singleton births are via cesarean delivery, so the distribution of births by gestational age is highly influenced by current medical standards.

Research from the mid-2000s suggests that mothers and infants are at higher risk of birth complications if pregnancy is allowed to extend beyond 42 weeks, so current practices encourage the use of medical interventions to ensure delivery before 42 weeks. This likely explains the substantial drop in post-term births between 1990 and 2011.

The data: Distribution of singleton births by gestational age
2011 2010 2006 2005 2000 1990
Under 28 weeks 0.6% 0.6% 0.6% 0.6% 0.6% 0.6%
28-31 weeks 1.0% 1.0% 1.0% 1.0% 1.0% 1.1%
32-33 weeks 1.2% 1.2% 1.3% 1.3% 1.2% 1.2%
34-36 weeks 7.3% 7.5% 8.1% 8.1% 7.3% 6.8%
37-38 weeks 25.7% 26.7% 28.9% 28.3% 24.4% 19.4%
39 weeks 30.1% 29.1% 26.2% 26.0% 24.9% 22.0%
40 weeks 19.9% 19.7% 19.4% 19.8% 21.9% 23.0%
41 weeks 8.6% 8.5% 8.6% 8.9% 11.3% 14.4%
42+ weeks 5.7% 5.6% 5.8% 6.0% 7.5% 11.5%
Source: CDC, National Vital Statistics Reports, Births: Final Data for 2011

Keep these stats in mind the next time you're betting in an office "guess the birth date" pool.

Have questions about the data?
Send me a message!

*Births prior to 37 weeks are considered to be pre-term (or premature). Births at or after 42 weeks are considered to be post-term. The wide range in the middle, from 37 to 42 weeks, truly reflects full-term gestation.

**Singleton births are births to an individual child, and excludes twins, triplets, or other multiple births.

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