Epidemiology of preterm birth and its clinical subtypes

Preterm birth (<37 weeks) complicates 12.5% of all deliveries in the USA, and remains the leading cause of perinatal mortality and morbidity, accounting for as many as 75% of perinatal deaths. Despite the recent temporal increase in preterm birth, efforts to understand the problem of prematurity have met with little success. This may be attributable to the under-appreciation of the etiologic heterogeneity of preterm birth as well as the heterogeneity in its underlying clinical presentations--spontaneous onset of labor, preterm premature rupture of membranes, and medically indicated preterm birth. In this paper, we review data regarding preterm births with particular focus on its incidence, temporal trends, and recurrence. Studies of births from the USA indicate that the recent temporal increase in the overall preterm birth rate is driven by an impressive concomitant increase in medically indicated preterm birth. However, the largest temporal decline in perinatal mortality has also occurred among medically indicated preterm births (relative to other clinical subtypes), suggesting that these obstetric interventions at preterm gestational ages are associated with a reduction in perinatal mortality. Recent data indicate that spontaneous preterm birth is not only associated with increased recurrence of spontaneous, but also medically indicated, preterm birth, and vice versa. This suggests that the clinical subtypes may share common underlying etiologies. Since medically indicated preterm birth accounts for as many as 40% of all preterm births, efforts to understand the reasons for such interventions and their impact on short- and long-term morbidity in newborns is compelling. Further research is necessary in order to understand the mechanisms and etiology of preterm birth, thus leading to the possibility of effective preventive or therapeutic strategies.

Defining the problem: the epidemiology of preterm birth

Preterm birth is the major clinical problem associated with perinatal mortality, serious neonatal morbidity and moderate to severe childhood disability in prosperous countries. Its prevalence is affected by the way in which gestational age is assessed, by national differences in the registration of births, associated practices, such as burial costs, or maternity benefits, which encourage or discourage registration, and by the perceived viability of extremely preterm infants. Despite these uncertainties, there is reliable evidence that preterm births are increasing, especially births before 28 weeks gestation. Contributing factors include births following assisted reproductive therapy and ovulation induction, especially multiple births, and the increasing proportion of births among women >34 years. On the other hand, improvements in neonatal care have substantially increased the survival of preterm infants during the last 15 years. There is wider acceptance of the importance of infection as a factor in preterm birth, and increasing recognition that processes leading to preterm birth may be initiated in very early pregnancy (the initiation of pre-eclampsia, major birth defects, premature placental separation), or even prior to pregnancy (prior pregnancy losses). It is unclear whether the familiar clinical presentations of preterm labour and birth reflect different pathophysiological processes. The pathways which link those processes to the consistent pattern of social differences in the probability of preterm birth have prompted new research approaches but in 2002 'the stubborn challenge of preterm birth' remains just that.

Classification and heterogeneity of preterm birth

Three main conditions explain preterm birth: medically indicated (iatrogenic) preterm birth (25%; 18.7-35.2%), preterm premature rupture of membranes (PPROM) (25%; 7.1-51.2%) and spontaneous (idiopathic) preterm birth (50%; 23.2-64.1%). The majority of multiple pregnancies (10% of all preterm births) are delivered preterm (50% for medical reasons). Although medical indications relate more to feto-maternal conditions, PPROM to infections and idiopathic preterm birth to lifestyle, these risk factors are identified in any category, emphasising that preterm birth has a multifactorial origin. Still, several incidences of preterm birth are not completely explained with a plausible cause for PPROM or spontaneous preterm labour suggesting that other causes have yet to be identified. In addition, preterm birth is associated with unrecognised severe congenital anomalies. Variability within the main categories may be explained by the studied population, ethnic group, social class and preventive interventions towards reducing spontaneous preterm birth where the proportion of medically-indicated preterm birth is increased. Despite being retrospective a classification according to gestational age at birth is important for neonatal prognosis. Preterm birth is stratified into mild preterm (32-36 weeks), very preterm (28-31 weeks) and extremely preterm (<28 weeks) with increasing neonatal mortality and morbidity. Recent studies suggested that infection was mostly responsible for extreme preterm birth, while stress and lifestyle accounted for mild preterm birth, and a mixture of both conditions contributed to very preterm birth.

Comparative accuracy of clinical estimate versus menstrual gestational age in computerized birth certificates

OBJECTIVE

This study compares gestational age data obtained by clinical estimate with data calculated from the date of the last menstrual period (LMP) as recorded on birth certificates.

METHODS

The authors analyzed 476,034 computerized birth records from three overlap years, that is, those that contained both menstrual and clinical estimates of gestational age, concentrating on cases within the biologically plausible range of 20-44 weeks.

RESULTS

The overall exact concordance between the two measurements was 46%. For +1 week it was 78%, and for +2 weeks it was 87%. Incidence of prematurity was 16% with menstrual gestational age, while it was 12% with clinical estimate. About 47% of the LMP-based preterm births were classified as term by clinical estimate. Eighty-three percent of clinical estimate-based preterms were also preterms by LMP-based gestation. Birthweight frequency distribution curves for LMP-based gestational age are bimodal, indicating probable miscoding of term births. An apparent over-representation of births coded as exactly 40 weeks by clinical estimate suggests rounding off near term for this method.

CONCLUSION

Agreement between menstrual and clinical estimates of gestational age occurs most often close to term, with significant disagreement in preterm and postterm births. Use of different methods of determining gestation in different years or geographic populations will result in artifactual differences in important indicators such as prematurity rate.

Defining preterm delivery--the epidemiology of clinical presentation

It is possible that preterm delivery is not a single entity but a cluster of conditions with different aetiologies that ultimately result in the delivery of an infant before 37 completed weeks of gestation. Whereas some researchers have reported aetiological heterogeneity, others have found no differences between subtypes or have disputed the desirability and utility of classifying preterm birth into subtypes. This study explores the relationship of maternal risk factors to type of preterm delivery in a cohort of over 7000 black and white women delivering singleton infants at the University of California, San Francisco's Moffitt Hospital between 1980 and 1990. Although the magnitude of the effect of individual risk factors differed between preterm delivery subtypes, the set of risk factors significantly associated with both categories of spontaneous preterm delivery was identical, while that associated with medically indicated preterm births was different. This study indicates that whereas the distinction between spontaneous preterm deliveries and those that are medically indicated seems valid, distinguishing between types of spontaneous preterm births may not lead to useful aetiological inferences.

The role of life events in different categories of preterm birth in a group of women with previous poor pregnancy outcome

The aim of the present study was to determine whether mothers reporting more life events experience more preterm births following both complicated and uncomplicated pregnancies. A Life Events Inventory was administered prospectively to women at high risk for poor obstetric and neonatal outcomes who took part in the Pregnancy Home Visiting Program (PHVP), a randomized controlled trial of the effect of a programme of antenatal home visits by midwives on the incidence of preterm birth. This study took place in Western Australia in the years 1984-1987. All women in the study had had a previous poor pregnancy outcome. The women were classified into two groups--those with complicated and those with uncomplicated pregnancies. Pregnancies classified as 'complicated' were defined as a pregnancy in which there was antenatal hospital admission(s) for hypertension, antepartum haemorrhage or other medical reason except for preterm birth. Pregnancies classified as 'uncomplicated' refer to all pregnancies without these complications. No significant association was found between life events and preterm birth although the total stress score for women with uncomplicated pregnancies almost reached significance, as did the number of life events for both women with complicated and women with uncomplicated pregnancies considered together. Life events were not shown to have a predictive relationship to preterm birth even when stratified by etiologically different groups. However, although stress was not an important predictor of preterm birth in this group of women at biological risk it may yet be so in a group at social risk.

[Etiologic classification of premature labor and its importance for prevention]

A retrospective analysis of 202 premature deliveries before 37 weeks was performed to identify major pathologies related to preterm delivery. The most frequent pathologies were premature rupture of membranes (32.4%), premature labour without recognisable aetiology (19.1%), hypertensive diseases in pregnancy (15.6%), multiple pregnancies (14.4%), malformations (9.8%) and bleeding in the 3rd trimester (6.4%). The majority of premature deliveries are related to 4 major pathogenetic disturbances: infection, problems of placentation, pathology of the foetus, pathology of the uterus. Each of these pathologies can lead to premature delivery via premature labour, premature rupture of membranes or termination of pregnancy for foetal or maternal pathology. In one third of premature labour, in another preterm premature rupture of membranes with labour after a variable latent period led to delivery, and in the remaining third, delivery was performed by a primary caesarean section or induction of labour for foetal or maternal pathology. Less than 25% cases were considered as failures of tocolytic treatment. Tocolytics, steroids or antibiotics, may help to improve the survival-rate in particular with very low birth-weight infants at less than 30 weeks gestation. A decrease in the overall rate of prematurity can be achieved only by a general improvement of the socio-economic working and living conditions of the female population, in particular of pregnant women.

Classification of threatened premature labor related to treatment with a prostaglandin inhibitor: indomethacin

68 patients complaining of premature uterine contractions were treated with a prostaglandin synthetase inhibitor--indomethacin. This agent had a marked effect on uterine contractions suppressing them completely after 1--8 h from the administration of the drug. Classificaiton of the premature contractions according to a scoring system method is analyzed in this study. The score classified the patients into three groups and enabled us to treat them with a correct dosage and period according to the clinical findings at admission of the patients. Indomethacin treatment was effective in all the 68 patients. The drug was well tolerated by all the patients with minimal side effects. The outcome of the newborns was satisfactory, and laboratory tests on various systems of the pregnant women and newborns were normal during and after treatment with indomethacin.