Assessing the risk factors before pregnancy of preterm births in Iran: a population-based case-control study

Preterm birth is defined as delivery before the gestational week 37 or day 259 [1–3]. It has been the most concerning complication among pregnant women and affects 10% of all pregnancies. Annually, 1 million neonatal deaths occur due to preterm birth [4]. It constitutes a large proportion of medical expenses and impose enormous economic burden on health care systems, families, and children [1]. Preterm birth is still a prevalent public health issue responsible for high perinatal mortality and long-term morbidity worldwide Despite improved perinatal care programs, it still remains a major leading cause of perinatal mortality, particularly in developing regions [5, 6].

Preterm birth is a multifactorial phenomenon, partially in association with immunologic, genetic, and environmental factors; however, its attributing factors have not yet been well studied [7, 8]. Previous studies concerned with preterm birth indicated that 45–50% of causes are unknown, 30% can be attributed to premature rupture of the membrane, and 15–20% are medical indications such as elective labor [8–10]. Recent studies have suggested that preterm birth is an independent risk factor for future cardiovascular diseases, cardiac ischemic diseases, and stroke [11, 12]. Due to the enormous economic and emotional burden of preterm birth and its associated complications, this study was conducted to examine the association between prenatal risk factors and preterm birth.

This population-based case-control study was conducted on 2463 mothers, including 668 cases and 1795 controls, referred to a health care center in several provinces of Iran, namely, Fars, Hormozgan, Kermanshah, Hamadan, Kohgiloyeh, and Boyerahmad, Yazd, Southern Khorasan, Golestan, and city of Mashhad (Fig. 1). A rural health care center is a health facility in a village that provides health care for approximately 9000 people of that village and several neighboring villages. Health care providers at a rural health care center include a general physician and public health and midwifery experts. The rural health care center supervises and supports health care facilities in villages and is linked to its superior urban health care center. An urban health care center is a health facility in cities providing care to approximately 12,500 people. Health care providers, including a general physician and public health and midwifery experts, provide laboratory, pharmaceutical, radiological, and medical care in the urban health care centers. Experts at rural and urban health care centers register the provided health care to every family in their health records, such as health care for pregnant women. However, the registered information in the family’s health records were insufficient, and hence we collected additional data through interviews with the study participants.

The case group was defined as women who had preterm birth in a recent pregnancy, and the control group was defined as women who had full-term birth in a recent pregnancy [13–15]. The sample size ratio in the control and case groups was 3:1. Data were collected through interviews according to a check list containing demographic information (mother’s age, ethnicity, occupation and level of education, place of residence, and consanguineous marriage) and information on the previous pregnancies (the outcome of previous pregnancy, mode of delivery, and interpregnancy interval).

Study subjects were recruited through a multistage cluster sampling method. In the first stage regarding geographical divisions of Iran, nine clusters (provinces) were randomly selected. In the second stage, in each of the nine clusters (provinces), four clusters (cities) were randomly selected from the north, south, east, west, and central areas. In each city, two health care centers (one urban and one rural health care center) were randomly selected. In each health care center, 10 check lists were filled in by well-trained interviewers according to a protocol. In each center, data collection process was conducted simultaneously on the same day for cases and controls. Data of the control group were collected from a random sample of mothers referring to the health care center. If < 10 cases were available in each health care center, the remaining check lists were filled in the nearest center, and if there were > 10 cases, the check lists were filled in for a random sample of mothers. We tried to maintain the same size for the case and control groups.

According to literature review, considering mother’s age > 35 years as a risk factor (p₀ = 0.3, p₁ = 0.44, z_0.95 = 2, z_(1-β) = 0.8, design effect = 2) [16] and using the proportion determination formula, the sample size was estimated as 370 for each study group. In this study, the association between preterm birth and 14 independent variables was evaluated. Therefore, taking into account an additional 20 samples for each independent variable, the total sample was calculated as 650 for each study group. The sample size was sufficient considering > 80% power of study.

This study was conducted on 2463 mothers referred to health care centers (668 cases with a history of preterm birth and 1795 controls without a history of preterm birth). The mean gestational age at the time of birth was 31.5.” 4.03 vs. 38.8 ± 1.06 weeks for the case and control groups, respectively.

The etiology of preterm birth has been a major concern in obstetrics worldwide. The cause of 50% of preterm births is unknown [18]. However, this study revealed a strong association between preterm birth and a history of abortion and stillbirth, ethnicity, interpregnancy interval, cycles of menstruation period, and consanguineous marriage. Consistent with other studies in this area, our study suggests an increased risk of preterm birth for mothers older than 35 years [18–22]. Martin et al. found an increased risk of preterm birth associated with older ages in women of high economic status [23]. The majority of studies indicated that the increased risk of preterm birth associated with increasing mother’s age may be confounded by socioeconomic factors or health complications associated with older ages, namely, hypertension, and renal diseases. Preventive strategies for older age mothers include providing appropriate health education and consultation, regular perinatal care during pregnancy, and encouraging mothers toward seeking effective family health [11, 22, 24]. Along with other studies, our study results suggest an association between preterm birth and ethnicity [4, 13, 15, 25]. Among all the studied ethnic groups, Balooch ethnicity was associated with an increased risk of preterm birth. This increased risk can be attributed to low socioeconomic status, high reproductive rates, low reproductive health status, insufficient reproductive knowledge, and poor nutrition. Preterm birth is affected by differences in ethnic groups regarding parents’ level of education, tobacco use, distress, and unfavorable experiences in life [13, 14, 26].

We observed an increased risk of preterm birth associated with consanguineous marriage, which was consistent with the results of other studies examining the genetic risk factors [27]. In addition, it should be considered that preterm birth can be affected by different environmental factors as well. Several studies suggested that even in the absence of genetic factors, preterm birth is associated with environmental factors such as socioeconomic status and tobacco use [27, 28]. Moreover, a number of studies suggested an interaction between preterm birth and parental genes or inheritance of human leukocyte antigen [29, 30].

Consistent with other studies, the present study demonstrated that a history of abortion and stillbirth is associated with an increased risk of preterm birth. Recent studies suggested an association between history of abortion and increased risk of preterm birth in subsequent pregnancies [11, 18, 31]. In addition, three large, population-based historical cohort studies and two large, case-control studies suggested that a history of abortion is a risk factor for preterm birth [31–33]. Several studies found an increased risk of preterm birth in association with more abortions and also indicated that various genetic and environmental factors can lead to repeated abortions [34–36]. It was also observed that abortion as the result of the last pregnancy is associated with an increased risk of preterm birth in gestational weeks < 32. The strength of this association decreases with increasing gestational weeks [31]. Although we observed an association between preterm birth and history of abortion, a population-based study in Pakistan reported conflicting results. Such inconsistent results can be attributed to the differences in the methods or limitations such as using data extracted from registry systems, lack of a control group, different definitions of gestational weeks for abortion, lack of control on potential biases, and confounding effects [31, 37].

We found that mothers with irregular cycles of menstruation period compared to those with regular cycles were more likely to develop preterm birth. Bonessen et al. also showed that regular cycles of menstruation period are associated with lower risk of prolonged pregnancy. In women with regular cycles of menstruation, the exact gestational age is clear and health care providers are not concerned with induction of pain [38].

Consistent with the results of other studies, our study results suggest an increased risk of preterm birth in mothers with < 1-year interpregnancy interval [39, 40]. Adams et al. suggested an increased risk of preterm birth associated with a 6- to 11-month interpregnancy interval. This risk decreased for interpregnancy intervals of > 47 months [41]. Krymko et al. [39] suggested that this association can be attributed to the presence of intrauterine infections before pregnancy or acute infections during pregnancy, mother’s physical weakness, emotional status, hormone secretion due to distress, or uterus contractions.

Preterm birth is a multifactorial issue in obstetrics. Despite technological improvements in the health care system, it still remains a major concern for health officials. In the present study, ethnicity, history of abortion and stillbirth, irregular cycles of menstruation period, consanguineous marriage, and narrow interpregnancy intervals were found to be risk factors for preterm birth. Regarding preventive strategies, it is recommended that mothers be provided with reproductive health care before and during pregnancy, particularly in the high-risk groups, to reduce the proportion of preventable cases of preterm birth.