Preterm birth is the single most important cause of perinatal mortality in North America and Europe (1). In addition, it is responsible for nearly half of all cases of congenital neurological disability, including cerebral palsy (2). Most of the affected babies have lifelong impairment and impose a significant economic burden on society. The primary prevention of preterm birth is therefore a major public health goal.
The identification of modifiable causal factors is an essential first step in any primary prevention programme. A short interpregnancy interval has been identified as potentially being such a risk factor but the results of epidemiological studies have been equivocal. Most were conducted in the USA using large administrative data sets (3-11) or large cohorts assembled for research purposes (12, 13), and found that a short interpregnancy interval was significantly related to an increased risk of preterm birth; however, the relative risks or odds ratios were 2 or less (3, 4, 6, 8-11). Some studies have found no significant relationship (5, 7, 12, 13). Similar results have been obtained in two studies in other countries (14, 15). Since most large data sets contain limited information on potential confounders, the findings of such studies could well be affected by confounding. Only one study excluded iatrogenic preterm births (12), only one excluded births of infants with congenital anomalies (15), and none excluded births to mothers with chronic medical conditions or obstetric complications, although all of these types of preterm birth could arise from an etiological pathway differing from those of most preterm births.
Recent studies in Denmark (16) and India (17) provided more convincing evidence of a possible causal relationship. More detail was available to control for possible confounders, the odds ratios were larger (3.6 and 2.7 respectively for very short interpregnancy intervals), and there was a risk gradient between very short and short interpregnancy intervals. However, each of these studies was conducted in a heterogeneous population and only a limited number of potential confounders could be identified and controlled for in the analysis. Also, iatrogenic preterm births, births involving congenital anomalies, and births to mothers with chronic disease or obstetric complications were not excluded.
We are unaware of any previous research on the relationship between interpregnancy interval and preterm birth in Emirati women. Our objective was to identify the relationship between interpregnancy interval and spontaneous preterm birth in a homogeneous population of healthy mothers delivering healthy babies and to control for as many potential confounders as possible in order to improve sensitivity and increase confidence that any significant relationship found was not attributable to confounding.
The study was carried out in Al Ain, United Arab Emirates, a small desert city served by three hospitals in which 99% of all deliveries occur. The general population is heterogeneous, consisting of Emirati people and expatriates from many countries. However, within this population, the citizens of the United Arab Emirates, readily identifiable from health records, are homogeneous. Most maintain their traditional Arab and Islamic culture while using the services of the modern health care system. Smoking and the use of alcohol are uncommon among women. All women who delivered are married and have reasonable socioeconomic status and access to health care (18). According to a survey conducted in 1995, the total fertility rate was 4.9 (18). Large families are not universal and there is a wide range of interpregnancy intervals.
Approval for the study was obtained from the Research Ethics Committee of the Faculty of Medicine and Health Sciences, United Arab Emirates University.
Methods case definition
A case was defined as a healthy multiparous Emirati woman with no obstetric complications who gave birth spontaneously to a healthy singleton before the beginning of the 37th week of pregnancy, as documented in delivery room records. …