Risk factors of early and late preterm birth

Preterm birth (PTB) is one of the major causes of morbidity in newborn. The aim of this study was to estimate the prevalence and to compare the associated risk factors of early and late PTB. This was a hospital-based cross sectional study conducted in 2016/2017. Women, 271, having PTB at 28-33+6 weeks period of gestation was taken as early PTB and 34-36+6 were taken as late PTB. Data was collected using semi-structured questionnaire, patients’ record book, adopting face- to - face interview technique and clinical examination. The annual prevalence of PTB was 7.25% of which 11% were <28+0 weeks, early PTB was 32% and late PTB was 57%. Mother with school education were 2.0 times more likely to have early preterm births than those having higher education (P-value: 0.005, COR: 2.061,95% CI:1.234-3.441). Mothers with positive history of PTB in any of previous pregnancy was 10.7 times more likely to have early PTB in current pregnancy (P-value: <0.001, COR: 10.677, 95% CI: 2.792 – 40.746). Both variables were found to have independent risk on early PTB in logistic regression analysis (education: P-value: 0.027, aOR: 2.973, 95% CI: 1.132- 3.047; previous history of PTB: P-value: 0.002, aOR: 9.191, 95% CI: 2.308 - 36.596). Early and late PTB have differential risk factors. Mothers with positive history of PTB and having lower level of education were more likely to have early PTB.


Introduction
Preterm birth (PTB) is defined as childbirth occurring at less than 37 completed weeks (259 days of gestation), counting from the last day of last menstrual period in women with regular menstrual cycles. 1 PTB is classified into early (<34 weeks) and late PTB (34-36 +6 weeks). 2 Over 60% of 15 million global PTB occurred in sub-Saharan Africa and South Asia. 3 The prevalence of PTB in Nepal ranged from 17% -19.5%. 4,5 The risk factors are socio-economic, environmental, lifestyle and obstetric factors. 6 Not all the babies born preterm are equally immature. Hence, early and late preterm babies need to be managed differently. For the differential management of the problem, we need to distinguish the risk factors early and late PTB. With our best knowledge there are very limited studies to compare risk factors of early versus late PTB. This study compared the epidemiological and obstetric risk factors of early and late PTB.

Materials and Methods
A cross sectional study was conducted among women with preterm birth (28-36 +6 ) week period of gestation) at Department of Obstetrics and Gynecology, a tertiary level referral center located in the capital city of Nepal. The study was conducted for 1 year from April 2016 to April 2017 and all the cases in the study period meeting selection criteria were enrolled. The objective of the study was to calculate the prevalence rate and compare the risk factors associated with early and late PTB. PTB occurring from 28 to 33 +6 weeks of POG were classified as early and those from 34 to 36 +6 weeks were classified as late based on Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) in 2005 AD. 2 The patients without detail antenatal record, IUFDs and unconfirmed POG cases were excluded. The independent epidemiological variables included were: maternal age, maternal ethnic group, maternal body mass index, maternal occupation and maternal education status. Similarly, the obstetrical variables were: parity, malpresentation, previous preterm birth, multiple pregnancies and inter pregnancy interval.
Semi-structured tool was designed to collect data which was pretested in 10% of samples. The data from pretesting was excluded at the final analysis. Confirmation of period of gestation was done by calculating from last menstrual period (LMP) by Naegeli's Formula.
LMP was considered reliable if she had history of regular cycle, specially last three cycles preceding the current pregnancy. Among those with irregular cycles, period of gestation was calculated according to first trimester USG or first trimester per vaginal examination and if not present then period of gestation was calculated according to BALLARD SCORE of the baby and if it suggests preterm then only the case was included for the study. Data was collected using face to face interview technique from postnatal ward. Patients' chart and hospital record books were also consulted for detailed information required for the study.
Data was cleaned and entered in MS Excel 10 and analyzed using SPSS 19. Frequency, percentage, mean and standard deviation; chi-square test, odds ratio were calculated as appropriate. Results with p value of less than 0.05 were taken as significant.
The proposal was ethically cleared from Institutional Review Board, Institute of Medicine. Informed consent was obtained from each participant. Confidentiality of the response was maintained throughout the study.

A. Prevalence of Preterm Birth in TUTH during
Study Period: Out of total 4838 births (including twins), 351 were PTB resulting into 7.25 % prevalence of PTB in TUTH in 2016/017. Out of total 351 preterm births, 37 (11%) were extremely preterm (<28+0 weeks) who were excluded in the study, 114 (32%) were early and remaining 200 (57%) were late PTB. In the early and late PTB 22 and 21 cases respectively were excluded due to unconfirmed gestational age. Ninety-two cases of early and 179 cases of late PTB were included in the study.

B. Distribution Epidemiological and Obstetrical Variables among Participants: Epidemiological
Variables: The maximum numbers of women were in age group 20-24 years and 25-29 years in both groups. Similarly, 6.0% percentage of participants was elderly gravida in this study. The mean age of mothers in early preterm birth category was 26.72±4.80 years and that of late preterm birth was 25.91±4.86 years.
Similarly, most of women were from Indo-Aryan ethnic group contributing to 70% of participants. Majority (80%) of women had BMI < 25 kg/m 2 . Most women were in moderate occupation group in both early and late preterm category (69.6% and 64.3%). Most of both early and late preterm groups had school level of education.
Most of the women (58%) were primiparous. Likewise, 94% of the births were having cephalic presentation. The percentage of multiple pregnancies in both early and late PTB was around 15%. All multiple births were twins in this study. Among early PTB slightly more than one quarter (28.2%) were having inter pregnancy interval <2 years. The same for late PTB were 18.7%.
Bivariate analysis of epidemiological risk factors was done using chi-square test for categorical variables. Out of all epidemiological variables, mothers with only school level education were 2 times more likely to have early preterm birth in comparison to those having higher level of   School level of education and history of preterm birth in any of the previous pregnancy were independently associated with occurrence of early PTB than late preterm birth. Women with school level education had 2.97 times risk of having early preterm birth than those with higher secondary and above education level (P-value: 0.027 aOR: 2.973 95%CI: 1.132-7.808).
Women with previous history of preterm birth had 9.19 times risk of having early preterm birth than those with no previous history of preterm birth (P-value: 0.002 aOR: 9.191 95%CI: 2.308-36.596). However, fetal presentation and occupation had no any statistical significance.   This might be because of better health system in reference study.

A. Prevalence of Preterm
This finding is lesser than estimated by WHO in 2010 4 and study done by Shrestha S et al. 5 This could be because the present study is done at tertiary care center located in capital city. Also, the reference studies were done nearly a decade back.
Early PTB comprises 32% and late 57% of total preterm birth. Remaining 11% were <28+0 weeks. This finding is lower than the findings from study in USA 2 and by McIntire DD. 10  Percentage of multiple pregnancies in both early and late PTB is around 15% (15.2% and 15.6%). The risk of early and late PTB in case of multiple pregnancies was almost similar. Among women who had inter pregnancy interval <2 years, there were more cases of early preterm birth than late preterm birth (28.2% vs. 18.7%). This is in opposition to the findings study in Missouri Department of Health by De Franco et al. 17 Our study was conducted in hospital which might not represent the actual burden of preterm birth in Nepal. Thus, we recommend conducting similar study in community setting which could reflect the exact status of the disease and the risk factors.

B. Epidemiological Risk Factors of Early and
In conclusion, lower maternal education and positive history of preterm birth are independent risk factors causing early preterm birth.

Source for this Research Fund: None
Conflict of Interest: None