New Definition of “Full-Term” Pregnancy Presents Other Problems

2013-10-23

By Emily Oster @ProfEmilyOsterOct. 22, 2013

Big news for women who are 38 weeks pregnant: you are no longer considered “full term.” The American College of Obstetricians and Gynecologists (ACOG) announced today that they would introduce a new classification system for the end of pregnancy.  Previously women who were 37 to 41 weeks pregnant were all considered “Full Term” and women 42+ weeks “Post-term”.  The new classification system calls for four groups:  “Early Term” (37-38 weeks), “Full Term” (39-40 weeks), “Late Term” (41 weeks) and “Post-term” (42+weeks).

I know, that’s a lot of terms. But the big change here is to separate out the 37 to 38 week period and acknowledge that, for most babies, this isn’t the ideal time to be born. Evidence has been piling up that babies born in this period do worse than babies born at 39 or 40 weeks. One very recent paper shows babies at 37 to 38 weeks are more likely than those born later to need time in the NICU, mechanical ventilation and IV fluids. But as induction and scheduled C-sections have become more common, the share of babies born at 37 or 38 weeks has risen from 17% in 1983 to 27% in 2009. Many of these deliveries are elective and driven, at least in part, by the perception by women (or their doctors) that things are ready to go at 37 weeks. The push to reduce these early-term deliveries has been going on for years.

This change in wording could make a big difference in practice. Early term births are expensive because of their added complications and by explicitly distinguishing them it makes it easier for insurers to refuse to cover elective births in this period. The problem, however, with having a strict guideline like this is that sometimes people start to treat it the cutoff itself as meaningful.

It is worth reflecting back on why this change is even necessary.  The idea that 37 weeks is “full term” is intended to be descriptive.  As pregnancy moves forward, the baby becomes more developed.  Among the last organs to develop are the lungs, and usually this happens by 37 weeks, so calling 37 weeks “full term” is simply an acknowledgement of this. But women, and their doctors, can start to think that when you move from 36 weeks 7 days pregnant to 37 weeks 0 days pregnant there is a magic switch which means your baby is ready.  This is obviously not the case.

This is illustrative of a broader issue, in medicine and elsewhere, with any kind of sharp rule-like cutoff.  In one example, researchers have shown that as a result of drawing the cutoff for very low birth weight at 1500 grams, babies born right below 1500 grams of weight actually have better outcomes than those right above, because they are given extra treatment.  Again, there is nothing biologically special about 1500 versus 1499.  The cutoff is descriptive, and when it’s treated as a rule, odd things happen. The same issues come up in finance, closer to my own field, when looking at lenders who used sharp cutoffs in credit scores in deciding who to lend to.  There is nothing special about a credit score of 620 compared to 619, and when you treat it like there is, you can get into trouble.

The worry, of course, to return to the babies, is that what is happening here is we are replacing one rule with another. The new guidelines fix one issue, but could lead doctors (in theory) to be too cautious about delivery in 37 or 38 weeks when medically warranted. It could also lead them to be too gung-ho about delivery as soon as you hit 39 weeks. The best case would be if women and their doctors took these terms as they are intended— as descriptive guidelines, not rules—and got better about letting nature take its course unless there is a medical reason not to.  If that happens, this truly will be a major change, and not just for the women at 38 weeks.