Preterm delivery has remained a major contributor to perinatal mortality and morbidity. Various strategies have, therefore, been adopted to prevent these births.
These strategies are stratified into levels from the first level of simple intervention to a tertiary level of complex actions.44
Level one measures are public health approaches that address all risk factors and affect the entire population of pregnant women. This includes improvement in socio-economic status of the people, nutrition, proper sex education, and prevention of teenage pregnancy, reduction in abortion, ensuring female education and adequate antenatal coverage.44 However, as the cause of preterm labour is usually not established, the success of these measures in preventing preterm delivery is modest.44
Level two actions are measures aimed at early detection of increased uterine contractility or of premature dilatation of the cervix. With prompt hospitalization and tocolysis, the pregnancy can usually be prolonged for a variable period of time.44 However, not all women who go into preterm labour meet current criteria for tocolytic therapy and tocolysis has not been proven to reduce the incidence of preterm delivery in defined population.44 There may be need, therefore, to predict preterm labour if preterm delivery is to be prevented.44 This has led to various studies aiming to predict preterm delivery. Krupa et al150 did a systematic review to assess published scientific evidence on preterm birth predictors. They performed an internet search for predictors of preterm births and evaluated the evidence level of each method. They found strong evidence that preterm births can be predicted using vaginal sonography to evaluate cervical characteristics, fetal fibronectin in cervicovaginal secretions and interleukin-6 in amniotic fluid. They found consistent evidence that digital cervical examination is a weak predictor of preterm birth. They found scanty evidence about the predictive ability of maternal history and perceptions of symptoms.
Mercer et al151 developed a risk assessment system for the prediction of spontaneous preterm delivery using clinical information available at 23 to 24 weeks' gestation and tested its predictive value. The system identified a minority of women who had spontaneous preterm deliveries. Among nulliparous women, the sensitivity of the system in predicting preterm delivery was 24.2% with a positive predictive value of 28.6%. In multiparous women, the sensitivity was 18.2% and a positive predictive value of 33.3%. The researchers concluded that the graded risk assessment system did not identify most women who subsequently delivered preterm. Various
other scoring systems have been developed to identify patients at increased risk for preterm birth. One of the more widely used is the Creasy scale modified and evaluated by Creasy and colleagues.152 A review of the existing risk score systems however shows that they have a poor positive predictive value and are thus in limited use.44
Maner et al153 studied whether delivery can be predicted using transabdominal uterine electromyography among 99 patients. They found that it could predict delivery within 24 hours at term and within 4 days among preterm babies. A home uterine monitoring device (abdominal tocodynamometer) has been demonstrated to be effective in detecting early preterm labour particularly in multiple gestations.154-157 This is based on the principle that there appears to be a window of a few days of increased uterine activity before true preterm labour begins. It is however expensive as it requires the equipment and transmission via a telephone modem to a central station for review of the tracing by a trained nurse. Thus, Eden et al158 tested the hypothesis that a provocative test for uterine contractility (the mammary stimulation test) at the beginning of the third trimester would reliably identify patients destined to be delivered before term. They evaluated uterine contractions among 94 gravid patients at risk for preterm delivery in response to nipple stimulation at a mean of 28.6 weeks' gestation. The mammary stimulation test was positive in 50% of patients tested and had a sensitivity of 84% and a positive predictive value of 34%. Ninety-four percent of patients predicted to be delivered at term actually were delivered at term (negative predictive value). A cost analysis suggested that the use of the mammary stimulation test could reduce the cost of ambulatory uterine activity
monitoring by nearly 50%. They concluded that if these findings could be validated in additional samples, the mammary stimulation test may be useful in prematurity prevention programmes.
Various signs observed during vaginal examination have been used as predictors of preterm delivery and are used to identify high risk women. These include a short cervix with length less than 1cm, dilated internal os defined as allowing the entry of a finger tip (about 1cm), a low presentation defined as the presenting part at the ischial spine, an expanded lower segment defined as shaping of the lower uterine segment with thinning of the supracervical area allowing easy palpation of the presenting part and uterine contraction felt by the patient as hardening.159 Measurement of cervical length manually and by vaginal ultrasound scan had been strongly predictive of preterm delivery.160 Mortensen et al161 however assessed the value of screening of cervical status in normal as well as in pregnancies with risk factors among 1327 pregnancies prospectively. They found the predictive value of a pathological cervix was 3 to 5-fold higher in the risk groups while cervical examination in low-risk pregnancies did not improve prediction of preterm delivery.
Hebbar and Samjhana162 in a randomized prospective longitudinal study involving 200 women assessed cervical length and internal os diameter with transvaginal ultrasound at 20 - 24 weeks and analyzed their ability to predict preterm birth. They found the risk of spontaneous preterm delivery increased steeply as cervical length decreased. At a cut off value of ≤ 2.5cm, the cervical length measurements had sensitivity, specificity, positive predictive value and negative predictive value of 77%, 95%, 56% and 98% respectively. They however found that the internal os diameter
lacked sensitivity and specificity. Berghella et al163 compared the accuracy of ultrasonographic and manual cervical examinations for the prediction of preterm delivery among 102 singleton pregnancies at high risk for preterm delivery by prospective follow up from 14 to 30 weeks with both serial cervical ultrasonography measurements and manual examinations of the length of the cervix. They found cervical length measured by ultrasonography as a better predictor of preterm delivery than cervical length measured by manual examination. They also found cervical ultrasonography in patients at high risk for preterm birth to be most predictive of preterm delivery when it is performed between 14 and 22 weeks' gestation. Andersen et al160 assessed 178 patients with singleton gestations and without cervical incompetence with transabdominal ultrasonography and endovaginal ultrasonographic cervical length measurement and manual vaginal examination of cervical length. They found that transabdominal ultrasonographic measurement of cervical length was not predictive of preterm delivery while endovaginal ultrasonographic cervical measurement predicted increased preterm delivery risk and concluded it may be superior to manual digital examination for preterm delivery risk assessment.
Chien et al164 evaluated the accuracy with which cervico-vaginal fetal fibronectin predicts preterm delivery using systematic quantitative overview of the available literature. They did an online search of MEDLINE database (1966 to April 1996), scanning bibliography of known primary and review articles and review of recent journal issues. Study selection, assessment of study quality and data extraction was performed in duplicate under masked conditions. They concluded that the presence of fetal fibronectin in cervico-vaginal mucus has limited accuracy in
predicting preterm delivery as the likelihood ratios for positive and negative test results generated only minimal to moderate changes in the pretest probability of preterm birth. Lockwood et al165 on the other hand examined whether serial assessment of cervical and vaginal fetal fibronectin allows for the prediction of preterm delivery in symptom-free patients. They obtained cervical and vaginal samples from 429 patients who received routine prenatal care between 24 and 37 weeks' gestation. They used a sensitive immunoassay to quantitate cervical and vaginal fetal fibronectin levels, and clinicians were blinded to fetal fibronectin results.
They concluded that among patients undergoing monthly cervical and vaginal sampling between 24 and 36 weeks' gestation, the presence of fetal fibronectin is a sensitive and specific predictor of preterm delivery. Heine, McGregor and Dullien166 compared the predictive accuracy of salivary oestriol levels with that of the modified Creasy score for predicting preterm delivery in a triple-blinded prospective trial conducted at 8 US health centres among 601 patients. They found salivary oestriol assessment was more accurate in predicting outcome than modified Creasy scoring.
Block and colleagues167 on the other hand evaluated if plasma oestradiol-17 beta and progesterone concentration values differed between patients who delivered before term and those who delivered at term. They measured their plasma concentrations serially in 90 patients at high risk of preterm delivery. The sensitivity of these measurements to predict preterm delivery was very low. They concluded that serial measurements of plasma oestradiol-17 beta and progesterone concentrations did not improve preterm delivery risk prediction.