Revisiting the 39 week rule

Street number sign on the wall

Over 3 years ago, I wrote about a study that showed that the “39 week rule” (banning elective deliveries before 39 weeks) increased the risk of stillbirth.

A new study purports to show that is not the case. The new study is Association of Widespread Adoption of the 39-Week Rule With Overall Mortality Due to Stillbirth and Infant Death. The authors confirm that the 39 week rule led to an increase in stillbirths BUT that was balanced by a decrease in infant death.

The authors claim that overall perinatal mortality rates were stable, but they used a non-standard definition of  perinatal mortality.

The new study is both comprehensive and carefully done, but it doesn’t actually settle the question.

The 39 week rule was designed to reduce neonatal intensive care unit admissions and save healthcare dollars. I argued against it for years for two reasons:

1. Given that we know that the stillbirth rate is higher at 39 weeks than at 37-38 weeks, implementation of the 39 week rule would increase term stillbirths.

2. The attempt to reduce perinatal morbidity from early term delivery is misguided. Sometimes the only way that you can prevent perinatal death is to deliver a baby early, which will result in increased morbidity like transient breathing problems and brief admissions to the NICU. An effort to reduce morbidity from early term delivery will NECESSARILY result in an increase in stillbirths.

A previous study Changes in the patterns and rates of term stillbirth in the USA following the adoption of the 39-week rule: a cause for concern? was presented at the 2016 annual meeting of the Society for Maternal-Fetal Medicine. As lead author James Nicholson, MD commented to Medscape:

This study raises the possibility that the 39-week rule may be causing serious unintended harm.

Term stillbirth is clearly one of worst obstetrical outcomes, and it occurs with relatively high frequency — in one per 1000 deliveries that reach 37 weeks …

Unless or until high-quality research is published that proves that the 39-week rule does not increase term stillbirth rates, the forced imposition of the 39-week rule should be immediately reconsidered.

The data presented by Dr. Nicholson and colleagues seemed pretty damning. In an effort to reduce mild, transient complications in newborns, we let nearly 300 babies die stillborn each year, exactly as critics of the 39 weeks rule predicted.

There was an extremely important caveat, however. A critical piece of data was missing and without it, it was difficult to draw conclusions.

What was missing? The perinatal mortality rate. If the 39 week rule is responsible for the increased stillbirth rate, the perinatal mortality rate should rise, too. If it didn’t rise, we’d have to consider the possibility that the babies who were stillborn would have died anyway after they were born and that the 39 week rule merely changed the timing of death, not the eventual outcome.

The new study attempts to address that issue:

Given the [previous] inconsistent findings, coupled with the policy goal of reducing adverse perinatal outcomes, this study examined the rate of stillbirth and infant death before and after the 2010 widespread adoption of the 39-week rule to determine the association with overall mortality. We hypothesized that the implementation of the 39-week rule may be associated with an increase in overall stillbirths, but that overall mortality—combined infant deaths and stillbirths—is reduced.

What did they find?

… Compared with the preadoption period, there was a decrease in the proportion of deliveries at 37 weeks (−0.06%) and 38 weeks (−2.5%) and an increase in the proportion of deliveries at 39 weeks (6.8%) and 40 weeks (0.2%) in the postadoption period (P < .001). The stillbirth rate increased in the postadoption cohort compared with preadoption (0.09% vs 0.10%; P < .001). The infant death rate decreased in the postadoption period compared with preadoption (0.21% vs 0.20%; P < .001). An overall mortality rate of 0.31% was calculated for the preadoption period and 0.30% for the postadoption period (P = .06). Additional analysis in a counterfactual model suggests that up to 34.2% of the difference in mortality could be associated with the 39-week rule.

Conclusions and Relevance: Stable overall perinatal mortality rates were observed in the 2-year period immediately after adoption of the 39-week rule, despite an increase in stillbirth.

At first glance, it appears that the inevitable increase in stillbirth was balance by a decrease in infant mortality. The authors claim that overall perinatal mortality rates were stable, but they used a non-standard definition of “perinatal mortality.” The actual definition of perinatal mortality is “stillbirths + neonatal deaths.” Their non-standard definition is “stillbirths + infant deaths.”

Why did they do that?

I obviously don’t know the authors’ thinking but I wouldn’t be surprised if when they initially ran the numbers, perinatal mortality rates increased because stillbirths increased. In other words, the 39 week rule, as predicted, appeared to be harmful. However, when they included deaths up to one year, the increased stillbirth rate was balanced by a decreased rate of infant death.

How did they justify their use of a non-standard definition of perinatal mortality?

…[N]ot all studies have used infant mortality, defined as death within 1 year of a live birth, despite evidence to suggest that sudden infant death syndrome rates that occur beyond the neonatal period may be associated with gestational age at delivery.

But that’s an assumption, not a fact.

Moreover, the background rate of infant mortality has been steadily decreasing for more than a century. Any decrease in infant mortality, therefore, may reflect an overall trend, not a result of the 39 week rule.

The authors acknowledge this limitation:

One critique of this work and other similar analyses is that the changes in mortality are owing to unknown confounding in the form of temporal changes associated with population differences, clinical practice, and administrative change. In our period examined, there are known and unknown temporal changes that add sources of confounding bias to our observed associations. First, there has been a steady decline in infant mortality in the United States during the period examined… We acknowledge that temporal changes are present and we have made efforts through a counterfactual model to minimize the outcome of such changes… When we considered mortality changes from gestational age redistribution alone through our counterfactual model, we estimated that up to 34.2% of the mortality reduction over time could be associated with widespread adoption of the 39-week rule.

But there are other issues that they don’t acknowledge.

First, they fail to calculate what proportion of decreased infant mortality is due to the “suggestion” that the 39 week rule prevents subsequent cases of SIDS. If it only accounts for few if any deaths, it doesn’t justify extending “perinatal” mortality beyond one month of age to one year of age.

Second, the 39 week rule was proposed as a way to reduce NICU admissions and thereby save healthcare dollars. The authors don’t investigate whether implementation of the 39 week rule did either.

The bottom line is that this study adds to the information we have about the 39 week rule, but it doesn’t resolve the issue.


  • mabelcruet

    Slightly OT, but can I ask a question of the obstetricians for my own education? SOL at term, mum known to be GBS positive urine a couple of weeks prior to delivery, so they covered in labour with intravenous antibiotics (penicillin as per RCOG protocol). Nothing obvious on CTG during labour but the baby was born unexpectedly flat with low Apgars and died at an hour of age. There’s very early chorioamnionitis in the placenta with funisitis indicating a fetal response, and I’ve got positive cultures for E. coli from stomach, spleen, heart blood and placenta, so the cause of death is infectious. At the morbidity and mortality meeting, someone said that in USA there was a rise in the number of E coli neonatal sepsis when GBS screening was introduced because the prophylaxis reduced the risk of neonatal GBS sepsis, so more babies died of other infections. I don’t quite understand-is it a real rise, or is it an apparent rise because of the reduced numbers of GBS cases? Or does prophylaxis allow for other organisms to take over, like overgrowth that you get in the bowel? Has anyone got any references handy?

  • fiftyfifty1

    Why not publish both rates: the perinatal rate using the standard definition, and the rate using their up-to-1-year modification? And then let the reader be the judge. Term stillbirth is a nightmare to me. So is SIDS of course, but at least with SIDS I am not powerless and as a parent I can decrease risk (no co-sleeping, no smoking, yes to pacifiers, back-to-sleep etc.)