Pediatric Respirology and Critical Care Medicine

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 3  |  Issue : 3  |  Page : 53--59

Preterm birth-associated factors analysis: A cross-sectional study in 2015


Yi-Hsin Yang1, Yen-Shan Yang1, Mei-Jy Jeng2, Ching-Yi Cho3, Yi-Hsuan Tang3, Yu-Hsuan Chen3, Chang-Ching Yeh4, Chung-Min Shen5,  
1 School of Medicine, Fu Jen Catholic University, New Taipei City, Taiwan
2 Department of Pediatrics, Taipei Veterans General Hospital; Institute of Emergency and Critical Care Medicine, School of Medicine, National Yang-Ming University; Department of Pediatrics, School of Medicine, National Yang-Ming University, Taipei, Taiwan
3 Department of Pediatrics, Taipei Veterans General Hospital; Department of Pediatrics, School of Medicine, National Yang-Ming University, Taipei, Taiwan
4 Department of Obstetrics and Gynecology, Taipei Veterans General Hospital, Taipei, Taiwan
5 School of Medicine, Fu Jen Catholic University, New Taipei City; Department of Pediatrics, Cathay General Hospital, Taipei, Taiwan

Correspondence Address:
Mei-Jy Jeng
Department of Pediatrics, Children's Medical Center, Taipei Veterans General Hospital, No. 201, Section 2, Shih-Pai Road, Taipei 11217
Taiwan

Abstract

Objective: The aim of this study is to investigate the current clinical factors associated with preterm birth in women delivering newborn infants in a tertiary medical center of a modern city. Methods: The medical records of women who delivered newborn infants in a tertiary medical center in Taipei city in 2015 were reviewed. To compare with the full-term group, the preterm group was defined by gestations of <37 weeks. Maternal characteristics, pregnant histories, underlying diseases, and peripartum conditions of enrolled mothers and the characteristics of their newborn infants were recorded and analyzed. Odds ratios (OR) were analyzed using logistic regression for factors associated with preterm deliveries. Results: A total of 1729 pregnant women (15–48 years) gave birth during the study period, including 1520 full-term and 209 (12.1%) preterm deliveries, accounting for 1778 newborns with 49 pairs of twins. After multivariate analysis, the following significant factors were found to be associated with preterm birth: multiple pregnancy (OR, 26.5; 95% confidence interval [CI], 12.7–55.4]), presence of maternal systemic lupus erythematosus (SLE) (OR, 10.4; 95% CI, 2.3–46.2), preeclampsia/eclampsia (OR, 7.6; 95% CI, 3.9–14.8), tocolysis requirement (OR, 6.6; 95% CI, 4.6–9.7), infection (OR, 2.4; 95% CI, 1.7–3.5), maternal diabetes (OR, 2.2; 95% CI, 1.0–4.4), and low maternal height (<155 cm) (OR, 2.2; 95% CI, 1.4–3.4). The preterm group also had more maternal blood loss (623 ± 543 vs. 399 ± 375 mL, P < 0.05) and a higher ratio of cesarean sections (59.3% vs. 26.8%, P < 0.05) than the full-term group. Conclusion: Multiple pregnancy, tocolysis requirement, lower maternal height (<155 cm), and the presence of maternal diseases during pregnancy, including SLE, preeclampsia/eclampsia, infection, and maternal diabetes, are significantly associated with preterm birth in Taipei city.



How to cite this article:
Yang YH, Yang YS, Jeng MJ, Cho CY, Tang YH, Chen YH, Yeh CC, Shen CM. Preterm birth-associated factors analysis: A cross-sectional study in 2015.Pediatr Respirol Crit Care Med 2019;3:53-59


How to cite this URL:
Yang YH, Yang YS, Jeng MJ, Cho CY, Tang YH, Chen YH, Yeh CC, Shen CM. Preterm birth-associated factors analysis: A cross-sectional study in 2015. Pediatr Respirol Crit Care Med [serial online] 2019 [cited 2020 Oct 28 ];3:53-59
Available from: https://www.prccm.org/text.asp?2019/3/3/53/292386


Full Text



 Introduction



Preterm birth of newborn infants is a major cause of neonatal mortality and morbidity. According to the World Health Organization, there are approximately 15 million babies born preterm every year, and the reported occurrence rate of preterm birth is 5%–18% worldwide.[1],[2],[3] In one recent review, the incidence rate of preterm birth was stated as approximately 11% of births.[3] Preterm birth is associated with a higher risk of developing adverse comorbidities and long-term outcomes, such as neurodevelopmental, cognitive, or physical problems.[2],[4],[5] Complications of preterm birth are the leading cause of death among children under 5 years of age.[1] Acute and chronic problems associated with preterm births not only cause infants to suffer but also burden their families.

The common maternal problems related to preterm birth include placental abnormalities; multiple gestations; maternal diabetes; preeclampsia; and preexisting conditions, such as high blood pressure and respiratory, heart, and thyroid diseases.[6] Other maternal factors have also been associated with preterm birth, including maternal age, body weight, height, or body mass index (BMI).[7],[8],[9],[10],[11] However, there is presumably a significant variation in these factors depending on the racial, health, and social-industrial developmental conditions of a particular region. Investigating regional risk factors related to preterm birth is important for establishing effective preventive policy in a specific region.

In recent decades, delayed childbearing has become a trend in developed countries.[12] Investigators have reported that mothers of advanced age are more likely to deliver preterm infants than younger mothers.[13],[14],[15],[16],[17],[18] In Taipei city, which is a representative modern city in northern Taiwan, many pregnant women are older than 30 years of age, and most complicated pregnancies are referred to as tertiary medical centers. Previously, the known premature birth rate in Taiwan was approximately 8%–10%,[16] and 75% of neonatal mortalities were related to very low-birthweight prematurity.[16] Further investigation of preterm births is important for future preventive medicine. This study was designed to investigate the current factors associated with preterm birth in women delivering newborn infants in a tertiary medical center of a modern city.

 Methods



This study was approved by the Institutional Review Board of Taipei Veterans General Hospital (VGHIRB 2016-12-006CC).

All live births from a medical center in Taipei between January 1, 2015 and December 3, 2015, were reviewed, and the mothers and their newborn infants were enrolled in this study. If babies were delivered at a gestational age of <37 weeks, they were placed into the preterm group. Otherwise, they were placed into the full-term group.

The basic characteristics, medical histories, and peripartum conditions of enrolled mothers, along with the basic characteristics of their newborns, were recorded and compared between preterm and full-term groups.

The recorded maternal characteristics included maternal age, height, weight, 6-month weight gain, BMI, multiple pregnancy, gravidity, and parity. The recorded medical histories of enrolled mothers included previous abortions, smoking during pregnancy, drinking during pregnancy, preeclampsia/eclampsia, maternal diabetes, systemic lupus erythematosus (SLE), thyroid disease, and uterine myoma. The recorded peripartum conditions included tocolysis treatment, Group B streptococcus (GBS) screening, fetal distress, premature rupture of membrane (PROM), preterm PROM (PPROM), delivery mode, amount of maternal blood loss, presence of meconium in amniotic fluid, and maternal infection. The recorded infant characteristics included gender, birth height, birth weight, gestational age, Apgar scores, placental weight, and duration of skin-to-skin contact with mothers.

The data were presented as mean ± standard deviation or n (%) as appropriate. Microsoft Office Excel 2016 (Microsoft Corporation, Redmond, WA, USA) and SPSS version 19.0 (SPSS Inc., Chicago, IL, USA) were used to perform data analyses. Graphs were made using SigmaPlot 12.0 (Systat Software Inc. San Jose, CA, USA).

A Student's t- test was used to compare numerical data between the two groups. A Chi-square test was used for proportion comparison between the two groups. A Chi-square with linear-by-linear association test was used to compare groups for ordinal categorical grouping, including maternal age, body height, BMI, and previous abortion frequency. Logistic regression was used for univariate and multivariate analyses for potential associated factors of preterm birth. A value of P < 0.05 was considered statistically significant.

 Results



In total, there were 1729 mothers enrolled in our study, including 49 with twin pregnancies. Among them, there were 209 mothers who experienced preterm birth, accounting for 12.1% of all enrolled cases, and they were categorized into the preterm group [Figure 1]a.{Figure 1}

The basic characteristics of the 1729 mothers enrolled are shown in [Table 1]. The ages of the mothers enrolled ranged from 15 to 48 years (mean, 33 ± 4 years), the body heights ranged from 138.5 to 177 cm (mean, 160 ± 5 cm), the body weights ranged from 37 to 121 kg (mean, 67 ± 10 kg), and BMIs ranged from 15.4 to 43.4 (mean, 26.1 ± 3.7). There was no significant difference in body weight, BMI, ethnicity, gravida, and the ratios of primiparas between the two groups.{Table 1}

There were significant trends showing higher ratios of preterm birth associated with the mother's older age [Figure 1]b, P = 0.042] and lower body height [Figure 1]c, P = 0.004] in a linear-by-linear association Chi-square test but with no significant trend in BMI [Figure 1]d, P = 0.136]. Comparisons based on mother's characteristics, proportions in the preterm group, were significantly higher than in the full-term group in the subgroup that included mother's age ≥40 years, height <155 cm, and BMI ≥30 (P < 0.05) [Table 1]. Furthermore, 6-month weight gain before delivery was significantly lower, and the proportion of multiple pregnancies was significantly higher in the preterm group (P < 0.05) [Table 1]. In the present cohort, all multiple pregnancies were twins.

Conditions of the enrolled mothers before and during this pregnancy are summarized in [Table 2]. Although the linear-by-linear association Chi-square test analysis of previous abortions was not statistically significant (P = 0.063), the proportion of previous abortions ≥3 times was significantly higher in the preterm group (P = 0.017). There were significantly higher proportions of mothers with pregnancy-related diseases (preeclampsia/eclampsia and maternal diabetes) and maternal diseases (maternal infection and SLE) (P < 0.05) in the preterm group when compared to the full-term group [Table 2]. The ratios of the tocolysis requirement and PROM ≥18 h were also significantly higher in the preterm group than in the full-term group [Table 2]. Conversely, the ratios of receiving GBS screening and having positive GBS screening tests were significantly lower in the preterm group (P < 0.05). There was no significant difference in maternal smoking, alcoholism during pregnancy, or the presence of fetal distress between the two groups (P > 0.05).{Table 2}

Univariate and multivariate analyses are shown in [Table 3]. The multivariate analysis results show that the top three associated factors were multiple pregnancy [odds ratio (OR), 26.5; P < 0.05], maternal SLE (OR, 10.4; P < 0.05), and maternal preeclampsia/eclampsia (OR, 7.6; P < 0.05) [Table 3]. Other significant associated factors included tocolysis requirement (OR, 6.6; P < 0.05), maternal infection (OR, 2.4; P < 0.05), maternal diabetes (OR, 2.2; P < 0.05), and body height < 155 cm (OR, 2.2; P < 0.05) [Table 3].{Table 3}

At delivery, mothers in the preterm group had significantly higher ratios of undergoing cesarean sections (49.3% vs. 22.9%, P < 0.05) and blood loss ≥500 ml during labor than in the full-term group (59.3% vs. 26.8%, P < 0.05) [Table 4]. Conversely, the ratios of spontaneous vaginal delivery, vacuum extraction, dysfunctional labor, and meconium stains in amniotic fluid were significantly lower in the preterm group [Table 4] (P < 0.05).{Table 4}

Characteristics of the 1778 delivered infants are shown in [Table 5], including 49 pairs of twins. Comparing infants of the preterm and full-term groups, the infants of preterm birth had a significantly higher proportion of twins, shorter body length, lower birth weight, lower Apgar scores at 1 and 5 min after birth, higher ratios of infants having Apgar score <6 at 5 min, lighter placental weight, and lower ratios of skin-to-skin contact right after delivery than in the full-term group (P < 0.05) [Table 5]. Comparing the duration of those having skin-to-skin contact with their mothers, there was no significant difference between the two groups [Table 5].{Table 5}

 Discussion



The study demonstrated a preterm birth rate of 12.1% among live births from a tertiary medical center in a modern city, Taipei city, in 2015. Among the mothers enrolled, multiple pregnancies, maternal diseases during pregnancy, tocolysis requirement, and lower maternal height were significantly associated with preterm birth. Significant preterm birth-related maternal problems were SLE, preeclampsia/eclampsia, infection, and maternal diabetes. Higher ratios of receiving cesarean sections and the amount of blood loss at delivery were also observed in the preterm group when compared to the full-term group.

A worldwide preterm birth rate of approximately 11% was reported by Vogel et al. in 2018 and Blencowe et al. in 2013.[2],[3] Artificially conceived, multiple pregnancies in developed countries accounted for many preterm births in the past decades.[11] Our analyzed incidence rate is slightly higher than that of the published reports because our data was collected in a tertiary medical center of a capital city, rather than on a nationwide scale. High-risk pregnant women are usually referred to as tertiary medical centers, which may contribute to the higher ratio of preterm deliveries observed in the present study.

Most of the preterm births cannot be directly attributed to any risk factors. Some of them are due to spontaneous onset of labor or PPROM, and others are due to provider-initiated induction of labor or elective cesarean section for maternal or fetal problems.[2] Investigators have previously reported that complications of placenta and cord, uterine over-distension, increased intrauterine volume, the prevalence of PPROM, and twin-to-twin transfusion syndrome may be risk factors for preterm birth in twin or multiple pregnancies.[11],[19] Similar to other reports, we demonstrated that multiple pregnancies, underlying diseases (including SLE and infections), pregnancy-induced preeclampsia/eclampsia or maternal diabetes, and tocolysis requirement were the main risk factors. Poor uterine situations not suitable for fetal growth in the diseased pregnant women could partially explain the mechanism of preterm births. Proper care for women with pregnancy-related diseases is important to reduce the risk of preterm birth.

Lower body height (<155 cm) of pregnant women was demonstrated as an independent risk factor for preterm birth in our study. This finding is compatible with the studies of Swedish women.[9],[10] In a study conducted on 192,432 Swedish women, Derraik et al. found a positive association between decreasing height and the likelihood of preterm birth.[10] In a meta-analysis study, Han et al. demonstrated that women of lower stature had a higher unadjusted risk of preterm birth. Although there may be other maternal conditions of interest, body height appears to be an easier measurement to obtain.[20] Because different races/ethnicities may have different height distributions, Shachar et al. investigated the influence of maternal height on preterm birth within various racial groups. An inverse association between maternal height and the risk of spontaneous preterm birth was found in Non-Hispanic whites and Asians, but not among blacks and Hispanics.[21] A study investigating pregnant women from low- to middle-income countries indicated that lower maternal stature is associated with poor infant conditions.[22] From a mechanical perspective, the lower maternal height may be caused by malnutrition, which influences maternal health and diverts resources for neonatal development away from the fetus.[23] However, in this study, this conclusion may not be applicable due to socioeconomic differences in the areas. Further investigations are required to understand this factor.

The mean age of pregnant women was 33 ± 4 years in the study in Taipei city, which is higher than in other areas.[12] In the past decades, delayed childbearing has become more common globally. Investigators have reported that pregnant women of both advanced age and young age should be regarded as risk groups for preterm deliveries.[13],[18] In this study, we did not find a significant relationship between young age and preterm birth, which may be due to the limited number of patients. We noted that the proportion of pregnant women of age ≥ 40 years was significantly higher in the preterm than in the full-term group, but the multivariate analysis did not identify age as an independent risk factor for preterm birth. The result of a cohort study has demonstrated that advanced maternal age is associated with increased miscarriage, preeclampsia, small infant size for gestational age, maternal diabetes, and cesarean sections.[17] Jacobsson et al. demonstrated that the risk of developing pregnancy-induced hypertension, severe preeclampsia, or placenta previa also increased with maternal age.[14] Our study results are compatible with their findings. Aged women usually have a higher possibility of developing pregnancy-related diseases and presumably a higher need for artificial fertilization that causes multiple pregnancies, both of which may increase the risk of preterm birth. Therefore, maternal problems in older women, rather than age itself, are the main reasons for preterm birth.

In addition to maternal diseases, poor nutrition is also a potential risk factor in undeveloped countries. Furthermore, maternal obesity has been related to preterm birth in developed countries.[24],[25] In Taipei city, poor nutrition is not a common problem for citizens. In the present study, we did not find a significant difference in women with BMI <18.5 between the preterm and full-term groups. One possible reason is that the sample population of BMI <18.5 was very small (n = 15) among the 1729 mothers enrolled. Conversely, we demonstrated that the proportion of maternal BMI ≥30 was significantly higher in the preterm than in the full-term group, but it was not identified as an independent risk factor after multivariate analysis.

Currently, obesity is recognized as an important health problem worldwide, including in pregnant women. The complex relationship between obesity and preterm birth has long been discussed.[26],[27],[28] In 2010, McDonald et al. conducted a meta-analysis to discuss the relationship between overweight mothers and preterm births.[27] They reported that there was an increase of induced preterm birth in overweight mothers.[27] Systemic inflammation, dyslipidemia, and multiple factors caused by obesity may contribute to the observed increase of preterm births.[24] Corresponding to our results, the comorbidity of obesity, as well as obesity per se, were the main causes for preterm birth reported by some investigators.[8],[28] Preeclampsia or eclampsia due to obesity, was suggested to play an important role.[25],[29],[30] Conversely, a negative correlation between overweight mothers and preterm births was reported by Ehrenberg et al., and they found that the decrease of uterine activity in obese women may be partially responsible.[7] Therefore, the role of obesity in preterm birth is most likely related to obesity-associated maternal problems and requires further exploration.

The enrolled women in the preterm group revealed a significantly lower rate of performance in GBS screening than those in the full-term group in our study. The main reason was the current policy in Taiwan advising pregnant women to undergo examination for GBS status during gestations of 35–37 weeks,[31] leading to a limited number of pregnant women with preterm birth available for the test. In a systematic review, an increase of positive GBS in preterm cases was reported.[32] Surve et al. also demonstrated that membrane vesicles secreted by GBS are the cause of chorioamnionitis, PPROM, and preterm births.[33] Therefore, the role of GBS infection in pregnant women requires continued attention.

Although we found some risk factors related to preterm birth in our cohort, we agree with Vogel's et al. opinion that the majority of preterm births occur in women without a clear risk factor.[3] Therefore, physicians still cannot neglect those pregnant women without risk factors for preterm birth and should provide proper care for newborn infants of preterm births to reduce their mortality and morbidity rates.[6]

There are several limitations to this study. First, this is a retrospective analysis; therefore, a few data are missing. Second, the GBS test was conducted less in the preterm group, and we could not adequately conclude the role of GBS in preterm births. Further investigation with a prospective, observational design may be helpful in future. Although we did not find distinct risk factors for preterm births compared to other studies, we have defined the current risk factors for preterm birth in a modern city, providing information for the health of women and infants.

 Conclusion



The preterm birth rate was 12.1% in the medical center in Taipei during the year of 2015. Multiple pregnancies; presence of maternal diseases during pregnancy, including preeclampsia/eclampsia, SLE, diabetes and infections; and lower maternal height (<155 cm) were significantly associated with a higher risk of preterm birth. Physicians should be alert when caring for pregnant women with these risk factors to prevent preterm deliveries and associated morbidities.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1World Health Organization. Preterm Birth. Available From: http://www.who.int/mediacentre/factsheets/fs363/en/. [Last accessed on 2019 Oct 26].
2Blencowe H, Cousens S, Chou D, Oestergaard M, Say L, Moller AB, et al. Born too soon: The global epidemiology of 15 million preterm births. Reprod Health 2013;10 Suppl 1:S2.
3Vogel JP, Chawanpaiboon S, Moller AB, Watananirun K, Bonet M, Lumbiganon P. The global epidemiology of preterm birth. Best Pract Res Clin Obstet Gynaecol 2018;52:3-12.
4Mento G, Nosarti C. The case of late preterm birth: Sliding forwards the critical window for cognitive outcome risk. Transl Pediatr 2015;4:214-8.
5Houben E, Smits E, Pimenta JM, Black LK, Bezemer ID, Penning-van Beest FJ. Increased risk of morbidities and health-care utilisation in children born following preterm labour compared with full-term labour: A population-based study. J Paediatr Child Health 2019;55:446-53.
6Requejo J, Merialdi M, Althabe F, Keller M, Katz J, Menon R. Born too soon: Care during pregnancy and childbirth to reduce preterm deliveries and improve health outcomes of the preterm baby. Reprod Health 2013;10 Suppl 1:S4.
7Ehrenberg HM, Iams JD, Goldenberg RL, Newman RB, Weiner SJ, Sibai BM, et al. Maternal obesity, uterine activity, and the risk of spontaneous preterm birth. Obstet Gynecol 2009;113:48-52.
8Cnattingius S, Villamor E, Johansson S, Edstedt Bonamy AK, Persson M, Wikstrom AK, et al. Maternal obesity and risk of preterm delivery. JAMA 2013;309:2362-70.
9Derraik JG, Lundgren M, Cutfield WS, Ahlsson F. Maternal height and preterm birth: A study on 192,432 Swedish women. PLoS One 2016;11:e0154304.
10Derraik JG, Lundgren M, Cutfield WS, Ahlsson F. Association between preterm birth and lower adult height in women. Am J Epidemiol 2016;185:48-53.
11Offiah I, O'Donoghue K, Kenny L. Clinical risk factors for preterm birth In: Morrison J, editor. Preterm Birth-Mother and Child. Rijeka, Croatia: InTech.; 2012. p. 73-9. Available from: https://www.intechopen.com/books/preterm-birth-mother-and-child/clinical-risk-factors-for-preterm-birth. [Last accessed on 2020 Jul 08].
12Mathews TJ, Hamilton BE. Mean age of mother, 1970-2000. Natl Vital Stat Rep 2002;51:1-13.
13Waldenstrom U, Cnattingius S, Vixner L, Norman M. Advanced maternal age increases the risk of very preterm birth, irrespective of parity: A population-based register study. BJOG 2017;124:1235-44.
14Jacobsson B, Ladfors L, Milsom I. Advanced maternal age and adverse perinatal outcome. Obstet Gynecol 2004;104:727-33.
15Fuchs F, Monet B, Ducruet T, Chaillet N, Audibert F. Effect of maternal age on the risk of preterm birth: A large cohort study. PLoS One 2018;13:e0191002.
16Premature Baby Foundation of Taiwan. Available from: http://www.pbf.org.tw/enhtml/en-a01.asp. [Last accessed on 2019 Oct 26].
17Khalil A, Syngelaki A, Maiz N, Zinevich Y, Nicolaides KH. Maternal age and adverse pregnancy outcome: A cohort study. Ultrasound Obstet Gynecol 2013;42:634-43.
18Sohn K. The trend in the relationship of advanced maternal age to preterm birth and low birthweight. Eur J Contracept Reprod Health Care 2017;22:363-8.
19Sela HY, Simpson LL. Preterm premature rupture of membranes complicating twin pregnancy: Management considerations. Clin Obstet Gynecol 2011;54:321-9.
20Han Z, Lutsiv O, Mulla S, McDonald SD. Maternal height and the risk of preterm birth and low birth weight: A systematic review and meta-analyses. J Obstet Gynaecol Can 2012;34:721-46.
21Shachar BZ, Mayo JA, Lee HC, Carmichael SL, Stevenson DK, Shaw GM, et al. Effects of race/ethnicity and BMI on the association between height and risk for spontaneous preterm birth. Am J Obstet Gynecol 2015;213:700.e1-9.
22Ozaltin E, Hill K, Subramanian SV. Association of maternal stature with offspring mortality, underweight, and stunting in low- to middle-income countries. JAMA 2010;303:1507-16.
23Martorell R. Body Size, Adaptation and Function. Human Organization 1989;48:15-20.
24Catov JM, Bodnar LM, Ness RB, Barron SJ, Roberts JM. Inflammation and dyslipidemia related to risk of spontaneous preterm birth. Am J Epidemiol 2007;166:1312-9.
25Wolf M, Hubel CA, Lam C, Sampson M, Ecker JL, Ness RB, et al. Preeclampsia and future cardiovascular disease: Potential role of altered angiogenesis and insulin resistance. J Clin Endocrinol Metab 2004;89:6239-43.
26Shaw GM, Wise PH, Mayo J, Carmichael SL, Ley C, Lyell DJ, et al. Maternal prepregnancy body mass index and risk of spontaneous preterm birth. Paediatr Perinat Epidemiol 2014;28:302-11.
27McDonald SD, Han Z, Mulla S, Beyene J; Knowledge Synthesis Group. Overweight and obesity in mothers and risk of preterm birth and low birth weight infants: Systematic review and meta-analyses. BMJ 2010;341:c3428.
28Mariona FG. Perspectives in obesity and pregnancy. Womens Health (Lond) 2016;12:523-32.
29Salihu HM, De La Cruz C, Rahman S, August EM. Does maternal obesity cause preeclampsia? A systematic review of the evidence. Minerva Ginecol 2012;64:259-80.
30Smith GC, Shah I, Pell JP, Crossley JA, Dobbie R. Maternal Obesity in Early Pregnancy and Risk of Spontaneous and Elective Preterm Deliveries: A Retrospective Cohort Study. Am J Public Health 2007;97:157-62.
31Jennifer RV, Lesley M, Stephanie JS. Prevention of Perinatal Group B Streptococcal Disease: Revised Guidelines from CDC, 2010; 2010. Available from: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5910a1.htm. [Last accessed on 2018 Aug 20].
32Valkenburg-van den Berg AW, Sprij AJ, Dekker FW, Dörr PJ, Kanhai HH. Association between colonization with Group B Streptococcus and preterm delivery: A systematic review. Acta Obstet Gynecol Scand 2009;88:958-67.
33Surve MV, Anil A, Kamath KG, Bhutda S, Sthanam LK, Pradhan A, et al. Membrane vesicles of group B Streptococcus disrupt feto-maternal barrier leading to preterm birth. PLoS Pathog 2016;12:e1005816.