The most serious preterm cases are those called Early Preterm birth, or before 32 weeks of pregnancy which is normally 40 weeks long. Still about 500,000 premature infants are born in the us, for a staggering $26 billion dollar price tag for their care per year. Babies have about 11% chance of being born prematurely, and we want that under 1/10. In my home state of Illinois, the rate has remained stuck around 12%. California, New York, and Oregon are among states who have reached that goal of under 1/10 babies born premature. the far southern states of Alabama, Mississippi and Louisiana are reporting the highest rates of prematurity. In an original research article by Markam et al of The Ohio State University has noted that the use of newer more aggressive strategies to use progesterone vaginally, has reduced their prematurity rate. If you have had a prior premature baby, and are pregnant or planning pregnancy, see your gyno to discuss what strategies you can use to help prevent prematurity. although it has been a goal to reduce premature deliveries, or in the ob lingo, preterm deliveries, a bit less than 1/10 births are still born preterm, and most of these, about 75% of these are born between 34 and 36 6/7 weeks. These have been termed the "late preterm births." In fact there has been about a 20% increase of babies born at these weeks of pregnancies in the ten years studied from 1996 to 2006 in the United States.
The experts have not been able to turn this around because there have been so many causes of these deliveries. One expert, Dr. Cynthia Bannerman, from Columbia University thinks that we can keep moms with many medical problems pregnant longer than we did in the past. She feels that we have been intervening early when moms have elevated blood pressures and low fluid and twin pregnancies when we could wait longer and have babies delivered at full term which would be healthier. Dr. Suneet Chauhan in the Society of Maternal and Fetal Medicine Debates in Feb of 2011 argues that this is really not going to be able to make much change in our rate of these late preterm births. Because so many complications like sudden rupture of membranes or complications of the placenta cannot be predicted and mandate early delivery still occur, we will persist in having these birth problems. As with other problems in medicine, a one on one solution is often indicated. Become healthy leading into pregnancy, do what you and your obstetrician can do to maintain that health in pregnancy, and it will be up to science to help us understand the underlying and complex causes that have left us with high and rising rates of premature deliveries in the United States.
Gynecologists have come to realize the shorter a cervix is, for the given stage of pregnancy, the more at risk the mom is for preterm birth. Although studies have varied, the current definition for a very short cervix, in a pregnancy that might benefit from treatment with progesterone, would be to be under or equal to 20 mm at under 24 weeks of pregnancy.
Oddly we know very little about why some pregnancies are destined to go either premature or overdue. Factors including first babies, previous late pregnancies, obesity, and genetics are the most commonly associated factors with prolonged pregnancies over 42 weeks. But, it's really not known why a particular pregnancy doesn't deliver by the due date. In a study from Northwestern University School of Medicine, published in Obstetrics and Gynecology September 2015 they studied over 9000 women, having their first baby, and found that the longer the cervix at 18-24 weeks, the more likely the pregnancy was going to be over due. So it's back to the Goldilocks theory: not too short, not too long but just right. Rates over 4.91 cm at that stage of pregnancy were associated with the latest pregnancies. Routine measurement of the cervix in the second trimester is almost always done now. The screening has been intended to predict who may be at risk for premature delivery, but now we have a new perspective on the data! and this is what the Genomic and Proteomic Institute is trying to sort, to see if these things are determined by our genetics!