Preterm Birth Update
The past 2 decades have seen an in crease in preterm births (PTBs), particularly in the United States. Each year, more than 500,000 American babies are born preterm, with medically indicated PTB accounting for 33% of that number, said Rita W. Driggers, MD, Director of the Maternal-Fetal Medicine Fellowship Program at Washington Hospital Center and Associate Professor, Department of Obstetrics and Gynecology, Georgetown University School of Medicine, Washington, DC, during her presentation at the 2010 annual meeting of the American Congress of Obstetricians and Gynecologists (ACOG).
Defined as birth before 32 weeks gestation, PTB is not significantly prevalent in any one ethnicity—18.3% in blacks, 12.1% in Hispanics, and 11.6% in whites.
Its impact can be devastating. Of all babies born preterm, 20% die within the first year of life. Although PTBs comprise only 2% of all US births, they account for >50% of infant deaths. Long-term disabilities in infants born preterm include cerebral palsy, mental retardation, respiratory problems, and hearing and visual impairment.
The financial effects of PTB are staggering as well. The annual costs associated with PTB in the United States total >$26 billion. The average firstyear medical expenses for a full-term infant are $3325 compared with $32,325 for a preterm infant.
Predicting PTB
The key to preventing PTB is early identification of women at risk, Dr Driggers said. Risk factors include:
- Previous PTB—risk increases with the number of PTBs
- Multiple gestation—mean duration of pregnancy is 35 weeks for twins, 32 weeks for triplets, and 29 weeks for quadruplets
- Short cervix—predictive value influenced by estimated gestational age
- Uterine infection—more likely with earlier-onset PTB
- Previous cervical surgery
- Smoking
- Illicit drug use
- Low prepregnancy weight
- Poor nutritional status
- Short interval between pregnancies (<6 months).
Biomarkers for PTB can be measured by amniotic fluid; urine; cervical and vaginal secretions; blood; or saliva. However, no one biomarker is effective for predicting PTB in all women. Salivary estriol level testing can be helpful in predicting late PTB, but is not useful in identifying women at risk for early PTB. Although bacterial vaginosis is an independent risk factor for PTB, there are insufficient data to indicate a PTB reduction benefit from screening and treating for bacterial vaginosis, Dr Driggers said.
Fetal fibronectin (fFN) testing has a negative predictive value, which is useful in avoiding unnecessary interventions (Table 1). An adhesive between the chorion and decidua, fFN can serve as a marker for disruption of the chorioamnion and underlying decidua. Although fFN normally shows up in cervicovaginal secretions until 16 to 20 weeks of gestation and again during the third trimester, Dr Driggers said, testing for its presence between those times can be valuable. Avoid testing in women who are asymptomatic or at low risk of PTB, she added.
The predictive value of negative fFN test results for delivering before 37 weeks ranges from 69% to 92%; the negative predictive value for ruling out delivery within the next 2 weeks is >95%.
Measuring cervical length is another valuable tool in assessing the risk for PTB (Table 2). According to Dr Driggers, there is a continuous association between cervical length and PTB—the shorter the cervix, the higher the risk for PTB. The test should be conducted between 14 and 30 weeks gestation, when a normal cervical length ranges between 25 mm and 50 mm. “The nice thing about this is, it can be used on anyone,” she said.
Preventing PTB
A number of other factors can cause PTB. Infection or inflammation, physical or psychological stress, abnormal uterine bleeding, stretching of the uterus, and genetics can all contribute to PTB, Dr Driggers noted. Heredity can often play a part when a woman has had a previous PTB or the woman was born preterm; paternal genes are not known to have an effect on PTB.
Several interventions aimed at preventing PTB have been shown to be of little to no help, with some even carrying deleterious effects, Dr Driggers said. Bed rest, although widely prescribed by OB/GYNs, has not proved effective in preventing PTB; it has shown evidence of harmful effects, including stress, weight loss, and depression. “The truth is, it does not affect preterm labor,” Dr Driggers said.
Although hydration has been thought to reduce uterine contractility by bringing more blood to the uterus and by decreasing pituitary secretion of antidiuretic hormone and oxytocin, there are insufficient data to support it as a preventive measure for PTB.
Home uterine activity monitoring involves the use of a tocodynamometer, which records and relays daily uterine activity to a patient’s practitioner. The method is approved by the US Food and Drug Administration for use in women with a previous PTB, but it has not shown benefit in preventing PTB, and is not recommended by ACOG.
Progesterone is one intervention that has proved helpful in preventing PTB. Progesterone supplements should be given to women with singleton pregnancy and previous PTB caused by preterm premature rupture of membranes or preterm labor. Its use should also be considered for asymptomatic women with cervical length <15 mm, she said.
Cervical cerclage—a surgical procedure to stitch the cervix closed during pregnancy—is another option for PTB prevention. “Elective cerclage at 13 to 16 weeks should be offered for patients with 3 or more unexplained second trimester losses or PTBs,” Dr Driggers said. Cerclage should also be considered for patients determined to be at risk for PTB with cervical length <15 mm to 25 mm.
Drug Therapy
Antibiotics. “The good news is 80% of women who present with preterm labor will deliver at term,” Dr Driggers said. In addition, she noted that treating for PTB can help. Although antibiotics should not be used for the sole purpose of preventing PTB, antibiotic therapy is in order if a culture test is positive. Group B streptococcus, which can be passed to the infant during delivery, should be treated according to the Centers for Disease Control and Prevention’s guidelines, which call for penicillin G (5 million units for the first dose and 2.5 million units every 4 hours until delivery, administered intravenously), or alternately, ampicillin (2 g for first dose and 1 g every 4 hours intravenously until delivery).
Corticosteroids are the most effective intervention for PTB, decreasing the incidence of respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, and mortality (with betamethasone only) in preterm infants, Dr Driggers explained. Candidates for a single corticosteroid include all women between 24 and 34 weeks gestation who are at risk of PTB within 7 days. Repeat courses of corticosteroids, however, should only be used in women enrolled in clinical trials, she noted.
Tocolytic drugs can be useful in prolonging gestation 2 to 7 days; however, the benefits of delaying labor for such a period only appear applicable when a course of corticosteroids is needed or for the sake of geographical transport of the woman to where she is to give birth, Dr Driggers said. It is important to consider the health status of the mother and fetus when considering tocolysis. “Many times, people are in preterm labor because the baby needs to come out,” she explained.
The choice of tocolytic agents should be individually catered to a patient’s needs, and factors such as contraindications, drug side effects, and gestational age should be taken into account. For women presenting with preterm labor before 28 weeks gestation, magnesium sulfate can be useful to reduce cerebral palsy risk for the infant