Dr. Jeffrey King, Chair of OB/GYN,New York Medical College, gave a March of Dimes sponsored lecture on prematurity. His lecture hit many of the same points as Dr. Socol's lecture on Current Topics. Dr. King began his lecture by showing the increased rate of preterm births in the United States. In 1988, preterm births were 10.2% of the total births, whereas in 1999 they were 11.8% of the total births. The greatest risk factor for a preterm birth is a previous preterm birth. The increased rate of twins/triplets/quadruplets from infertility treatments has contributed to the increased number of preterm births. Twins have a 55% risk of being born prematurely and triplets have a 94% chance.
In the United States, there are 10,799 babies born each day, of which 1239 are born preterm; 817 babies are born daily weighing < 2500 g, 156 are born weighing < 1500 g, and 12 are born weighing < 454 g (1 pound). While preterm births are increasing as a whole, the group that is especially increasing is the very low birth weight group (VLBW), with weight < 1500 g. This group has a 70-fold increased rate of mortality in the first year of life.
Neonatologists have now pushed the envelope, saving some fetuses born at 22-23 weeks. According to Dr. King, female premature infants do better than males. For infants born at 24 weeks gestation, 50% of females survive whereas only 35% of males survive. African-American female infants do the best; white males have the worst survival. A recent abstract from the National Institute of Child Health and Human Development (NICHD) showed that survival for infants born weighing between 501 g and 1500 g has continued to improve: 84% of 4438 infants weighing 501 g to 1500 g at birth survived until discharge to home or a long-term care facility.
However, morbidity and major mortality remain high, 71%, in the smallest infants (< 600 g at birth). Although the survival rate of premature infants is improving, the morbidity associated with premature birth can be lifelong. School performance has been shown to increase with increased birth weight. Unfortunately, according to the NICHD study, 97% of all VLBW infants and 99% of infants weighing < 1000 g at birth had weights that fell below the 10th percentile at 36 weeks' postmenstrual age. In addition, the financial cost of survival is great. The average lifetime cost of a preterm infant exceeds $500,000. Respiratory distress syndrome and prematurity are 2 of the 3 most expensive conditions in medicine. (Spinal cord injury is the third.)
The following subsets of prematurity-related clinical research were discussed at ACOG: (1) cervical length, (2) cerclage, (3) infection with bacterial vaginosis or trichomonas, (4) fetal fibronectin, (5) salivary estriol, and (6) home uterine monitoring.
Cervical length can be a valuable tool to predict preterm labor in some patients. However, as with NT, the technique is subjective. Therefore, who performs the test is as important as the result. Cervical length must be done transvaginally with an empty bladder. Multiple images should be taken with the shortest one recorded. Perform over 5 minutes with fundal pressure. One recent study in JAMA showed that cervical length of < 25 mm at 16 to 18 weeks' gestation in women with a history of a prior preterm birth was a significant predictor of spontaneous preterm birth before 35 weeks gestation. In fact, Iams and colleagues demonstrated in 1996 that women with short cervixes had a greater relative risk of preterm delivery. Women at 24 weeks' gestation with a transvaginal cervical length of 1.3 mm (1st percentile) had a 14-fold relative risk of preterm delivery. Iams also demonstrated that cervical competence is a continuous rather than categoric variable, which can indirectly be evaluated by transvaginal sonography of the cervix.
The term cervical incompetence has been in the obstetric literature for many years and is generally reserved for women with multiple second-trimester miscarriages or preterm labor. With the advent of transvaginal ultrasonography, the question is whether funneling seen on sonogram is the same thing as cervical incompetence. A study by the Royal College of Obstetricians and Gynaecologists did show a benefit for the use of cerclage for cervical incompetence. However, in terms of decreasing preterm labor, the benefit was only seen in 1 in 25 patients offered cerclage. With this in mind, Dr. Rust did a study at Lehigh Valley Hospital in Allentown, Pennsylvania, randomizing women to cerclage or no cerclage on the basis of evidence of preterm dilation detected at midtrimester ultrasound. Unfortunately, he did not show a benefit in perinatal outcomes. In a follow-up study, Rust discovered that the sonographic findings of second-trimester internal os dilation, membrane prolapse, and distal cervical shortening likely represent a common pathway of several pathophysiologic processes.
In patients with a previous failed cerclage, does the type of cerclage matter? One retrospective cohort study showed that in patients with a failed transvaginal cerclage, transabdominal cerclage is associated with a lower incidence of preterm delivery and preterm premature rupture of membranes. However, transabdominal cerclage requires special training and is indicated only in select patients.
Infection is thought to be a major cause of preterm labor and delivery. Could something as common as bacterial vaginosis be responsible for preterm delivery? Recent studies have demonstrated that bacterial vaginosis is associated with the preterm delivery of low-birth weight infants independent of other risk factors. Among women with bacterial vaginosis who had a prior preterm delivery, the use of metronidazole plus erythromycin reduced the risk of recurrent preterm delivery. The next logical step would be to treat asymptomatic women with bacterial vaginosis. Unfortunately, the treatment of asymptomatic bacterial vaginosis in pregnant women did not reduce the occurrence of preterm delivery or other adverse perinatal outcomes.
Trichomonas vaginalis infection during pregnancy has also been associated with preterm delivery. However, a recent follow-up study did not find a benefit to treating asymptomatic Trichomonas vaginalis in terms of decreasing preterm delivery.
Other recent areas of clinical research in the prevention of preterm delivery have also been discouraging. Fetal fibronectin has gained some popularity in the United States as a tool to predict preterm labor. Fetal fibronectin has a high negative predictive value. However, Dr. King pointed out that it has a low positive predictive value of 13%. Basically, fetal fibronectin tells the obstetrician which patient is not going to deliver. These patients could then benefit from fewer interventions. However, the ideal screening test would tell the obstetrician which patient is likely to deliver. Fetal fibronectin is not able to do that.
Home uterine contraction monitoring has been shown to be of no benefit in past studies. It was recently again studied in TheNew England Journal of Medicine. Although the likelihood of preterm delivery increases with frequency of contractions, measurements of this frequency are not of benefit in predicting preterm delivery.
Another recent study to identify patients at risk for preterm labor dealt with salivary estriol. Unfortunately, this test, like fibronectin, has a low positive predictive value. After recent excitement, salivary estriol, too, has fallen out of favor.
Medscape Ob/Gyn. 2002;7(1) © 2002 Medscape
Cite this: Highlights in Obstetrics From the 50th Annual Meeting of The American College of Obstetricians and Gynecologists - Medscape - Jun 12, 2002.