Demographic changes account for most of the reduction in the preterm birth rate

Preterm birth is one of the hottest topics in obstetrics. A preterm birth, defined as delivery before 37 weeks of gestation, carries numerous risks to the newborn, some of which can have lifelong effects. For years, the preterm birth rate in the United States increased each year and practitioners seemed powerless at stopping it. The preterm birth rate rose inexorably from 9.4% in 1981 to 12.8% in 2006. Countless studies have probed the causes of this rise and ways to reduce the preterm birth rate.

Fortunately, in 2006, we started to see a decline in the preterm birth rate, down to 11.4% in 2013. Although it doesn't seem like much, a drop from 12.8% to 11.4% is equal to an 11% reduction, which equates to a lot of benefit for babies. In absolutes, the number of preterm births in 2013 was 542,893 infants. Our first instinct may be to congratulate ourselves on the tremendous impact of the introduction of evidence-based interventions for the prevention of preterm birth: progesterone treatments for certain at-risk groups and cerclage (a stitch in the cervix). However, according to a new opinion article from a group of OBGYNs from Jefferson University in Philadelphia, changes in demographic factors are responsible for the bulk of the reduction. Most specifically, teens and women older than 35 years of age are at the highest risk of preterm birth. The teenage birth rate has been decreasing since 1991; for instance, the rate was 10.2% in 2006, down to 6.9% in 2013. The reduction in teen pregnancy may be the number one factor for the overall reduction in the preterm birth rate these past few years.

On the other side of the age spectrum, more women older than 35 years of age are having children. However, the reduction from fewer teen pregnancies far outweighs the increase from more mothers aged over 35 years. The authors estimate that, between 2006 and 2012, teen mothers delivered 23,000 fewer preterm neonates while women aged 35-44 delivered only 9,000 additional preterm neonates.


The introduction of evidence-based guidelines for the use of progesterone and cerclage have, of course, had an impact. They could, however, have a larger impact. We now know that women with a history of a spontaneous preterm birth are at increased risk of a second preterm birth. If in their next pregnancy, these women receive a shot of progesterone (17P), we can reduce preterm births by 0.02%, or 10,000 births. In 2007, it was estimated that only 74% of obstetricians used 17P. For these same women at increased risk, if an ultrasound shows that their cervix has shortened to less than 25mm before 24 weeks of pregnancy, a cerclage could prevent 23,000 preterm births, reducing the rate by 0.22%. Finally, new research demonstrated that universal ultrasound screening for a short cervix, regardless of birth history, paired with treatment with vaginal progesterone for cervix less than 20mm, could result in 9,500 fewer preterm births.

Smoking is one area in which public policy has had a positive impact on the preterm birth rate. Smoking cessation programs for pregnant women and bans on public smoking (in the workplace or at bars and restaurants) are two great examples of life-saving public policy changes. So far smoking cessation programs account for only 0.02% reduction in the preterm birth rate. But- there is a 10.45% reduction in preterm birth in states with a smoking ban as compared with those without a ban. This represents a major source of improvement that we have yet to fully realize.

Finally, one other major risk factor for preterm birth is multiple gestations (twins, triplets, etc). Twins have a 65% preterm birth rate and triplets a 97% preterm birth rate. Reducing the number of higher order multiples (triplets and higher) through changes in in vitro fertilization methods is an important step towards reducing preterm births. However, because they account for such a small percentage of all births, this reduction does not account for a significant part of the total preterm birth rate. Reducing the number of twins from IVF would make a significant difference.

Taken all together, the number of factors that affect the preterm birth rate can be overwhelming. Mothers' ages, existing knowledge about useful interventions, rate of adoption of these interventions, smoking, twins, IVF... The take home message here is clear: Reducing the preterm birth rate is one of the most important efforts in obstetrics. The good news is that the rate is declining, thanks in large part to fewer teen pregnancies and in small part to the slow adoption of interventions like progesterone, cerclage, and smoking bans. The bad news is that we have a long way to go. Reducing the number of twin pregnancies from IVF, providing pregnant women with environments that are free of smoke, and treating 100% of women who could benefit from progesterone or cerclage are some of the most important next steps in continuing and accelerating the decline in the preterm birth rate.