Introduction

Preterm birth (PTB) is one of the major public health problems worldwide. According to the latest estimates, an average of 15 million babies are born prematurely each year globally, which is depressingly common (Deng et al. 2021). Even more worrisome is the fact that PTB rates are increasing in almost all countries with reliable data (Vogel et al. 2018). In China, an observational study of the incidence and temporal trends in PTB showed that the incidence of PTB was 6.1% and that it increased from 2012 to 2018 in both single and multiple pregnancies (Deng et al. 2021). With the implementation of the two-child policy and the increase in advanced maternal age (≥ 35 years), the PTB rate increased more significantly (Deng et al. 2021).

Although significant progress has been made in the treatment of preterm infants with the rapid development of medical technology in recent years, PTB is still considered to be an important cause of neonatal death and death of children under 5 years of age, and associated with a risk of morbidity in later life (Liu et al. 2015; Hossain et al. 2022). It is important to note that preterm infants may face many problems throughout their lives, with both short-term and long-term consequences. Short-term consequences include an increased risk of neonatal respiratory disease, sepsis, neurological disorders, feeding difficulties, and vision and hearing problems (Araújo et al. 2012; Mwaniki et al. 2012; Platt 2014). As children grow up, the most common consequences include more hospitalizations, poorer neurodevelopmental outcomes, learning difficulties, cognitive or developmental delays, and behavioral and socioemotional problems (Johnson et al. 2015; Moreira et al. 2014; Orchinik et al. 2011). These negative impacts have attracted the attention of many researchers, medical personnel, and pregnant women. Therefore, in order to prevent PTB as a growing global problem, it is necessary to conduct research on the incidence of and factors associated with PTB and to better understand the determinants of PTB during pregnancy and in previous history. This will enable the prevention of PTB and accelerate progress towards the target of halting the number of preterm deaths by 2025 (Black et al. 2010; Collaborators 2021).

Previous studies conducted in several different countries have focused on risk factors for PTB and found that the occurrence of PTB is associated with multiple factors. A study in Bangladesh showed that mothers who used the contraceptive pill were less likely to have a PTB, whereas those who had a high fever during the pregnancy period, had received antenatal care services less than four times, or had married for the first time before the age of 18 were more likely to have a PTB (Hossain et al. 2022). A community-based cohort study conducted in northern Uganda demonstrated that maternal human immunodeficiency virus (HIV) infection was a risk factor for PTB and that maternal education of 7 years or more was a protective factor for PTB (Odongkara et al. 2022). A large retrospective cohort study using the National Vital Statistics System database in the United States clarified that gestational diabetes mellitus (GDM) increases neonatal PTB (Deng et al. 2022). A population-based study in France found that a previous PTB, short stature, underweight, obesity, and a low education level were common risk factors for PTB (Delnord et al. 2018). Studies conducted in China between 2015 and 2016 showed that younger or older maternal age, overweight or obesity, maternal disease, hypertensive disorders in pregnancies, history of miscarriage or stillbirth, history of PTB, placenta previa, placental abruption, chorioamnionitis, fetal distress, multiple pregnancies, fetal anomaly, male fetus, and antepartum stillbirth were associated with an increased risk of PTB (Chen et al. 2019). A systematic review and meta-analysis found that lifting gross weight of more than 100 kg per day and standing for more than 2.5 hours per day were associated with an increased risk of PTB in women (Cai et al. 2019). From previous studies, it can be seen that most only briefly analyzed the influencing factors of PTB.

In order to achieve maternal and child health-related indicators of UN Sustainable Development Goal 3, namely to reduce the global maternal mortality ratio to less than 12 per 100,000 live births by 2030, there is an urgent need to focus on the burden of PTB and conduct detailed research on the modifiable risk factors associated with it (Odongkara et al. 2022). To the best of our knowledge, studies on PTB and the associated risk factors with large sample sizes in eastern China are limited. The incidence of PTB may vary by socioeconomic conditions and region, and each region should quantify the true burden of PTB in line with its local circumstances (Zhou et al. 2022). Therefore, we conducted a multicenter retrospective study based on the Jiangsu Maternal and Child Health Information System. The objective of the study was to assess the incidence of PTB in coastal areas of eastern China and to explore the factors associated with PTB in terms of pregnancy-related factors, health-related factors, and maternal characteristics. This will help clinical workers to carry out a comprehensive and reasonable assessment of the overall situation of pregnant women at an early stage, effectively identify the groups at high risk for PTB, and provide a scientific basis for the development of reasonable and targeted preventive measures and care programs.

Materials and methods

Study setting

This retrospective study was carried out in Lianyungang, Jiangsu Province, eastern China. Lianyungang, also known as Port City, is located in the northeast of Jiangsu Province, and is an underdeveloped coastal city. In the east, it faces Japan, South Korea, and North Korea across the sea. In the west, it connects Xuzhou and Suqian, in the south, Huai'an and Yancheng, and in the north, Rizhao and Linyi, Shandong. The city has three districts and three counties, covering a total area of 7615 square kilometers. In 2021, the permanent population was 4.602 million, and the birth population was 31,400. The per capita gross domestic product (GDP) was 81,015 yuan (about US$ 11,210), which was approximately in line with the national per capita GDP (80,976 yuan).

Data sources and study population

All data for this retrospective study were obtained from the Jiangsu Maternal and Child Health Information System, which provides the most complete data on maternal and birth outcomes in Lianyungang, Jiangsu Province, eastern China. We selected data related to 30,006 pregnant women who delivered in seven maternal and child health and family planning service centers, 114 township health centers, 18 public hospitals, and 10 private hospitals in Lianyungang, Jiangsu Province, eastern China, between 1 July 2021 and 30 June 2022. Of these, 199 pregnant women did not meet the inclusion criteria and were excluded. Finally, a total of 29,807 pregnant women were included in the study. The inclusion criteria for participation were as follows: (1) delivery at 28 weeks of gestation or later; (2) successful delivery and survival of the baby; (3) complete clinical data; and (4) permanent resident of Lianyungang (living in Lianyungang for 6 months or more) and those who lived in the area from pregnancy to delivery.

Measurement of variables

Outcome variables

The outcome variables in this study were PTB and non-PTB. PTB is defined as the birth of a live baby after 28 weeks of gestation but before 37 weeks of gestation (WHO 1977). Non-PTB was defined as full-term birth. Gestational age was calculated by subtracting the date of conception from the birth date of the infant. We calculated the incidence of PTB during the study period, from 1 July 2021 to 30 June 2022, as the number of PTBs divided by the total number of live births and expressed as percentages.

Pregnancy-related variables

Pregnancy-related variables included maternal age (≤ 18 years, 19–34 years, ≥ 35 years), prenatal screening (yes, no), mode of delivery (cesarean section, vaginal delivery), number of perinatal infants (1, ≥ 2), hemoglobin testing (yes, no), number of prenatal visits (< 5, ≥ 5), number of deliveries (1, 2, 3, ≥ 4), and mode of pregnancy (natural pregnancy, assisted pregnancy). Assisted pregnancy included artificial fertilization and test-tube babies.

Health-related variables

Health-related variables included hypertension (yes, no), abnormal amniotic fluid (yes, no), preeclampsia (yes, no), hepatitis B surface antigen (HBsAg) positivity (yes, no), syphilis infection (yes, no), GDM (yes, no), thyroid disease (yes, no), and placenta previa (yes, no). Hypertension was defined as blood pressure ≥ 140/90 mmHg during pregnancy. Abnormal amniotic fluid referred to polyhydramnios or oligohydramnios. GDM referred to glucose tolerance that occurred or was first recognized during pregnancy, with lower blood glucose levels than overt diabetes diagnosed outside pregnancy. Placenta previa is when the placenta is attached to the lower half of the uterus or covers the cervical opening.

Maternal characteristics

Maternal characteristics included history of ectopic pregnancy (yes, no), history of pelvic surgery (yes, no), history of hysteroscopic surgery (yes, no), birth spacing >5 years (yes, no), history of PTB (yes, no), history of cesarean section (yes, no), and history of miscarriage of all types (≥ 3 times, < 3 times).

Statistical analysis

The data were exported to Microsoft Excel (Microsoft Office Professional Plus 2016, Microsoft Corp., Redmond, WA, USA), cleaned, and imported into SPSS 21.0 (IBM Corporation, Armonk, NY, USA) for statistical analysis. Descriptive statistical analysis was used to describe pregnancy-related factors, health-related factors, and maternal characteristics. Mean and standard deviation (SD) were used to describe continuous data, and frequency and percentage were used to describe classified data. The chi-square test was used to compare the variations in PTB among the different groups. For variables with low theoretical frequency, we used Fisher’s exact tests. Variables with p < 0.05 in the univariate analysis were included in the binary logistic regression model to find the independent influencing factors of PTB after excluding potential confounders, which were expressed by odds ratio (OR) and 95% confidence interval (CI). Before binary logistic regression, we tested the collinearity between the independent variables. The results found that the variance inflation factor (VIF) of each variable was less than 10, and the tolerance was much greater than 0.1. Therefore, there was no collinearity between the independent variables. All statistical tests were conducted two-sided, and p < 0.05 was considered statistically significant.

Results

Incidence of PTB

A total of 29,807 eligible pregnant women were included in the study, of whom 1518 (5.1%) were PTB and 28,289 (94.9%) were full-term birth (Fig. 1).

Fig. 1
figure 1

Incidence of PTB

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Pregnancy-related factors and their relationship with PTB

In the current study, the average age of the study subjects was 28.89 ± 4.97 years, and most pregnant women (86.0%) were 19 to 34 years. A large proportion (90.7%) underwent prenatal screening, but nearly one in ten (9.3%) did not. Nearly half (42.7%) delivered by cesarean section, and a small number (1.2%) had had two or more perinatal births. Most (94.4%) had been tested for hemoglobin during pregnancy, while 1681 (5.6%) had not been tested. Among the study subjects, there were 1121 (3.8%) pregnant women with less than five prenatal visits, and 28,686 (96.2%) with five or more prenatal visits. More than three in five (62.9%) were giving birth for the first time, more than one in four (25.8%) were giving birth for the second time, and more than one in ten (11.2%) were giving birth for the third time or more. The majority (97.3%) had become pregnant naturally, and only 810 (2.7%) had become pregnant with assistance. The results of the univariate analysis showed that age, prenatal screening, mode of delivery, number of perinatal births, number of prenatal visits, number of deliveries, and pregnancy mode were significantly correlated with PTB (p < 0.001), and that the occurrence of PTB with varying hemoglobin testing conditions could not be considered to be different (p > 0.05) (Table 1).

Table 1 Pregnancy-related factors and their relationship with PTB
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Health-related factors and their association with PTB

Of the study subjects, 609 (2.0%) had hypertension, whereas most (98.0%) had blood pressure in the normal range. A large proportion (99.6%) had normal amniotic fluid, and only a small number had polyhydramnios or oligohydramnios (0.4%). A small number, 92 (0.3%), had preeclampsia, and 920 (3.1%) were HBsAg-positive. A small proportion (0.6%) had syphilis during pregnancy, while most (99.4%) did not have syphilis during pregnancy. Nearly 6% (5.6%) experienced some degree of abnormal glucose tolerance during pregnancy, whereas a large number (94.4%) did not have GDM. Of the 29,807 pregnant women, 1018 (3.4%) had thyroid disease and 397 (1.3%) had placenta previa. Univariate analysis found that there was a statistical correlation with PTB for hypertension, preeclampsia, HBsAg positivity, GDM, and placenta previa (p < 0.05) (Table 2).

Table 2 Health-related factors and their association with PTB
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Maternal characteristics and their association with PTB

Of the 29,807 pregnant women, 390 (1.3%) had a history of ectopic pregnancy, 435 (1.5%) had a history of pelvic surgery, and 169 (0.6%) had a history of hysteroscopic surgery. A small number (1.3%) had an interval of more than 5 years between the current pregnancy and the last birth. Most (99.7%) had no history of PTB, and only 94 (0.3%) had ever given birth prematurely. More than one in five (20.6%) had undergone a cesarean section, and a small percentage (1.6%) had a history of three or more miscarriages of any kind. Univariate analysis showed that history of ectopic pregnancy, PTB, and cesarean section were associated with PTB (p < 0.05) (Table 3).

Table 3 Maternal characteristics and their association with PTB
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Factors associated with PTB

Binary logistic regression analysis revealed that pregnant women who were aged 18 years or younger (OR: 1.706, 95% CI: 1.207–2.409), aged 35 years or older (OR: 1.519, 95% CI: 1.312–1.760), had delivered by cesarean section (OR: 1.542, 95% CI: 1.360–1.748), had had two or more perinatal births (OR: 14.557, 95% CI: 11.507–18.415), or had four or more deliveries (OR: 1.614, 95% CI: 1.192–2.186) were more likely to have PTB. However, those who had had five or more prenatal visits (OR: 0.299, 95% CI: 0.241–0.372) were less likely to have PTB. In addition, pregnant women with hypertension (OR: 2.184, 95% CI: 1.666–2.863), preeclampsia (OR: 4.948, 95% CI: 3.014–8.124), GDM (OR: 1.610, 95% CI: 1.328–1.952), placenta previa (OR: 2.543, 95% CI: 1.836–3.523), or a history of PTB (OR: 9.028, 95% CI: 5.667–14.383) were at increased risk of PTB (Table 4).

Table 4 Binary logistic regression analysis of factors associated with PTB
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Discussion

According to a World Health Organization (WHO) report in 2019, China had more than 1.1 million PTBs in 2014, equating to a rate of 6.9% (Chawanpaiboon et al. 2019). The incidence of PTB may vary in different regions due to socioeconomic conditions, geographical location, and living habits (Zou et al. 2014). Therefore, we studied the incidence of PTB and related factors in Lianyungang, Jiangsu Province, eastern China. We found that the incidence of PTB was 5.1%, which was lower than the rate of 10.3% in southwest China and 7.1% in northern China, but higher than the 2.3% in central China (Zou et al. 2014). Compared to studies in other countries, the incidence of PTB in our study was comparable to that in Uganda (5.0%) (Odongkara et al. 2022) and France (5.5%) (Delnord et al. 2018), and lower than that in Nepal (9.3%) (Gurung et al. 2020), Ethiopia (9.1%) (Walle et al. 2022), and the United States (7.92%) (Gao et al. 2022b). The above differences may be due to variations in design, socioeconomic status, and maternal population. In addition, most of the previous research data were derived from multiple medical institutions, whereas our data only included pregnant women in the Lianyungang area, which may also have caused some differences. Our findings indicate a high incidence of PTB in coastal areas of eastern China. PTB can lead to a range of adverse consequences, with most children born prematurely suffering lifelong complications such as learning disabilities, diabetes, and kidney disease (Ream and Lehwald 2018). It also leads to significant long-term health system costs and imposes a considerable psychological and financial burden on families (Petrou et al. 2009). However, the pathogenesis of PTB is multifactorial. It is critical to identify all recognized risk factors and select the most informative ones to enable the identification of accurate risk prediction models (Della Rosa et al. 2021).

Getting pregnant at the right age is key to reducing the risk of adverse pregnancy outcomes (Gao et al. 2022a). The current study found that pregnant women aged 18 years and younger and those aged 35 years and older had a higher risk of PTB than women aged 19 to 34 years. This finding is not unique to our report. Previous studies have also found that teenage motherhood and older motherhood are associated with an increased risk of PTB (Chen et al. 2019; Hossain et al. 2022; Kildea et al. 2017; Subedi et al. 2022). In China, since the implementation of the three-child policy and with the social trend towards later childbearing, more women are becoming pregnant at an older age, and they may face a higher risk of complications, thereby increasing the incidence of PTB (Vounzoulaki et al. 2020). Due to physiological factors, teenage mothers may not have a fully developed uterus and complete body function, and the risk of PTB will also increase. In addition, the number of women with advanced maternal age is currently increasing and is likely to continue to do so over the next few years (Deng et al. 2022). Therefore, we should give special attention to the health care of young and older women during pregnancy and the perinatal period.

Cesarean section rates have also increased in recent years (Gao et al. 2022a). Studies show that more than 40% of babies are born by cesarean section (Baran et al. 2022). Cesarean section increases the risk of spontaneous PTB in a subsequent pregnancy and is a cause of low birth weight (Walle et al. 2022; Zhou et al. 2022). Our study showed that women with cesarean section delivery had a higher rate of PTB than those with vaginal delivery. Univariate analysis also found that women with a history of cesarean section were more likely to have PTB. This finding is similar to results from other studies. Efforts to reduce cesarean section delivery, particularly elective or nonmedical ones, may reduce rates of PTB, which is important for public health (Yasseen et al. 2019). Multiple pregnancies are associated with an increased risk of PTB (Shah et al. 2014; Subedi et al. 2022). Our study also revealed that more than one perinatal birth was a risk factor for PTB. Previous studies have found a higher risk of PTB among nulliparous mothers (Gurung et al. 2020; Mayo et al. 2017). However, we found that pregnant women with four or more deliveries had a higher risk of PTB than those giving birth for the first time. Therefore, obstetric medical staff should focus on monitoring high-risk pregnant women with multiple pregnancies and four or more and deliveries, and promptly implement intervention measures. In the current study, 3.8% of pregnant women had fewer than five prenatal visits, and their incidence of PTB was significantly higher than that among women who had five or more visits, which is consistent with the results of previous studies (Gurung et al. 2020; Hossain et al. 2022; Muglia et al. 2022). Studies conducted in the United States have also shown that pregnant women who do not receive any antenatal care have a two to four times higher risk of adverse pregnancy outcomes (Taylor et al. 2005). Prenatal visits focus on the screening of high-risk pregnant women. They help to identify risk factors that may adversely affect the progress of pregnancy, and facilitate timely early intervention, such as antibiotic prescriptions for infections and dietary supplementation for malnutrition (Requejo et al. 2013). Pregnant women should be encouraged to visit a trained health provider for regular prenatal visits, thereby reducing the risk of PTB.

Hypertensive disorders affect 3% to 10% of pregnancies worldwide and are a leading cause of maternal morbidity and mortality (John et al. 2022). Our study suggests that pregnant women with hypertension are more likely to experience PTB. This finding is corroborated by previous studies, which have revealed that gestational hypertension is a significant predictor of PTB (Kildea et al. 2017). In addition, a prospective, population-based pregnancy cohort study in rural Nepal showed that high systolic and diastolic blood pressure in the third trimester was associated with an increased risk of PTB (Subedi et al. 2022). Hypertension during pregnancy can lead to fetal changes in trophoblast cell angiogenesis, apoptosis, oxidative stress, hypoxia, and placental development, leading to placental dysfunction and fetal intrauterine growth restriction, which may in turn lead to premature delivery (James and Nelson-Piercy 2004; Lu et al. 2018; Santana et al. 2018). Our study also shows that preeclampsia is a risk factor for PTB. This indicates the importance of measuring blood pressure during pregnancy. Pregnant women should be encouraged to measure blood pressure frequently, monitor the rise of blood pressure, and analyze its causes, since timely diagnosis and treatment are important for the prevention of PTB.

The time of pregnancy is a critical period that affects the health of the fetus and newborn. GDM is a common complication of pregnancy, affecting 13.9% of pregnancies worldwide (Gojnic et al. 2012). It refers to different degrees of abnormal glucose tolerance during pregnancy, often accompanied by significant changes in glucose and lipid metabolism, and is significantly associated with adverse obstetric outcomes (Gao et al. 2022a). GDM also increases the risk of type 2 diabetes and cardiovascular disease in the mother, as well as the likelihood of future obesity in the child (Plows et al. 2018; Szmuilowicz et al. 2019). In the past few years, the incidence of GDM has increased significantly, especially in advanced maternal age (Deng et al. 2022). The current study found that GDM was significantly associated with an increased risk of PTB. Previous studies have also observed significantly higher rates of fetal distress, polyhydramnios, and PTB in pregnant women with GDM compared with those without GDM (Deng et al. 2022; Gao et al. 2022a). This may be because, with the increase in maternal blood glucose, excessive glucose passes through the placenta into the fetus, resulting in fetal hyperglycemia and hypertonic diuresis, and increased urinary excretion, which leads to excessive amniotic fluid in pregnant women. In late pregnancy, the premature rupture of membranes can easily cause PTB (Aviram et al. 2016; Hiersch et al. 2019; Kong et al. 2019). Therefore, close attention must be paid to GDM to reduce the risk of PTB through high-quality early care or treatment.

Previous studies have documented an increased risk of adverse intrapartum, postpartum, and fetal outcomes in pregnant women with placenta previa, including preterm birth and cesarean section (Jauniaux and Bhide 2017; Jauniaux et al. 2021). Placental abnormalities have also been shown to mediate the association between advanced maternal age and PTB (Li et al. 2022). Our study also identified placenta previa as a risk factor for PTB. Women with placenta previa are more likely to have vaginal bleeding, leading to an increased risk for PTB (Jauniaux and Bhide 2017). Intervention strategies should focus on improving the health status of pregnant women to reduce the risk of placental abnormalities. The risk of PTB is generally thought to be inversely related to gestational age (Goldenberg et al. 2008). The current study showed that women who had already experienced PTB were more than nine times more likely to have PTB than those who had not. Previous studies conducted in remote communities in northern Australia and in France have also shown that prior PTB is the strongest single predictor of recurrent PTB (Kildea et al. 2017; Delnord et al. 2018). Therefore, in the course of clinical work, more attention should be paid to pregnant women with a history of PTB to reduce the incidence of PTB recurrence.

Limitations

This study has some limitations that should be clarified. First, maternal weight changes, intrahepatic cholestasis, fetal growth restriction, and rupture of membranes were not included in our study because of incomplete data. Previous studies have found that these factors are associated with PTB, and future studies need to analyze them (Kildea et al. 2017; Subedi et al. 2022; Yang et al. 2022). In addition, health-related lifestyle variables such as smoking need further study. Second, our study did not distinguish well between the type of cesarean section, namely planned or emergency, and the severity of maternal illness. Third, due to the retrospective nature of this study, there may be unavoidable information bias. Fourth, this study was conducted in only one region, and the findings are only applicable to regions sharing similar social and economic backgrounds. Fifth, due to data limitations, this study did not distinguish between iatrogenic PTB and spontaneous PTB. In conclusion, despite these limitations, this study aimed to provide important information on the incidence and risk factors of PTB in coastal areas of eastern China. We believe that these results will have important implications for the management of PTB to improve neonatal outcomes and reduce morbidity and health care costs.

Conclusion

This study investigated the incidence of PTB and its associated factors. The incidence of PTB is high in coastal areas of eastern China. Age less than or equal to 18 years, more than or equal to 35 years, cesarean section delivery, two or more perinatal births, four or more deliveries, less than five prenatal visits, hypertension, preeclampsia, GDM, placenta previa, and history of PTB are important predictors of PTB. Therefore, awareness of the problem of PTB in pregnant women should be raised, and reasonable and targeted preventive measures and care programs aimed at reducing the incidence of PTB should be formulated. At the same time, more attention should be paid to pregnant women with risk factors for PTB and modifiable factors associated with PTB in order to reduce the risk of PTB, improve neonatal outcomes, and achieve maternal and child health-related indicators of UN Sustainable Development Goal 3 as soon as possible.