n={Z_1-α/2 √(2P ̅(1-P ̅ ) )+Z_1-β √(P₁(1-P₁)+P₂(1-P₂) )}²/(P₁-P₂)²

^{P1 =} (OR×P₂)/(1+P₂(OR-1) )

P ̅=(P₁+P₂)/2

P₂= Proportion of exposure in control group

P₁= Proportion of exposure in cases

OR= Odds ratio

α = Significance level

1-β= Power

Taking a case control ratio of 1:1, 230 cases and 230 controls were recruited for the present study (Table 1).

Selection of cases and controls

All consecutive cases as per the case definition and satisfying the inclusion - exclusion criteria were included in the study till the sample size was attained. Women as per the definition criteria for controls and those coming next to every case was selected as controls.

ASSOCIATION OF PRETERM BIRTH AND FAMILY HISTORY OF PRETERM LABOUR

Family history of preterm birth

The study examined the association between family history of preterm birth and present preterm birth. More than 90% of cases and controls had no family history of preterm birth. When comparing between cases and controls, the percentage of cases with a family history of preterm birth was higher than that of control group and this difference in percentage was found to be statistically significant by using the chi-square test. Odds ratio of 3.9 (p value 0.010 and 95% CI=1.3-12.1) indicates that the chance of preterm birth was nearly 4 times more among mothers who had a family history of preterm birth (Table 2 and Figure 1).

Data Analysis 

The data was analysed using descriptive and inferential statistics. The distribution of variables were looked into and appropriate statistical significance test were undertaken. Collected data was entered in MS EXCEL. Completeness was checked and analysis was done using downloaded version of statistical software SPSS version 20. Qualitative variables were expressed using proportions and quantitative variables in mean and standard deviation. Quantitative variables were categorized using suitable cut offs and analysed appropriately.

Test of association was done using Chi square test and Fisher‘s exact test. Odds ratio and 95% confidence interval was computed as the estimate of strength of risk. Logistic regression (Backward stepwise method) was done as multivariable analysis to find out the significant determinants after finding the collinearity of the variables.

RESULTS

A case control study was undertaken to identify the various determinants of preterm delivery, during the period 1st November 2012 to 30th November 2013 at SAT hospital, Thiruvananthapuram. A total of 460 subjects were studied; 230 cases, women delivered between 24-37 weeks of gestation and 230 controls, women who delivered after 37 weeks but before 42 weeks. Distribution of study variables in relation to case-control status were analysed. Chi-square test was used for finding the association between the variables in the present study. Strength of association of significant variables were presented using odds ratios with 95% confidence interval after dichotomously categorizing them. This was followed by the presentation of logistic regression analysis to find out the significant determinants

DISCUSSION

The role of maternal genetic study in foetal health and pregnancy outcome is not clear [1]. Carried a study to investigate familial recurrence of preterm labor and analyse the relative contributions of maternal and foetal genotypes and the risk of preterm delivery. From the study, more than 90% of cases and controls had no family history of preterm birth. When comparing between cases and controls, the percentage of cases with a family history of preterm birth was higher than that of control group and this difference in percentage was found to be statistically significant by using the chi-square test. Odds ratio of 3.9 indicates that the chance of preterm birth was nearly 4 times more among mothers who had a family history of preterm birth. In a study from 1974, the sisters of women who delivered preterm appeared to be almost twice as likely as sisters-in-law to themselves deliver prematurely [2]. Mother’s family history could pose risk for preterm birth, according to a study by researchers from Ben-Gurion university of the Negev and Soroka University Medical Centre. The study published in American Journal of Perinatology followed 2300 mothers and daughters over 22 years and found that the risk of preterm delivery was significantly higher among 34 percent women whose mothers had a history of preterm birth. In the present study, family history of preterm birth came out to be a significant determinant with an adjusted Odds ratio of 9.693 with (95% CI of 2.037- 46.128, p value=0.004). The strongest established risk factor for PTD is a previous PTD (7), but women who themselves were born preterm or whose sisters delivered prematurely have also been shown to be at increased risk of delivering preterm (8-11). Despite recognition of a familial component to PTD, the strongest established risk factor for PTD is a previous PTD (7), but women who themselves were born preterm or whose sisters delivered prematurely have also been shown to be at increased risk of delivering preterm (8-11). Despite recognition of a familial component to PTD, the strongest established risk factor for PTD is a previous PTD (7), but women who themselves were born preterm or whose sisters delivered prematurely have also been shown to be at in-creased risk of delivering preterm (8–11). Despite recognition of a familial component to PTD, n the strongest risk factor for preterm labour is previous preterm labour but the women who are themselves born preterm or whose sisters delivered prematurely have also been shown to be at increased risk of delivering preterm [3-6].

CONCLUSION

Preterm birth is a complex trait with a significant familial component. So from the study it is established that if a pregnant women has a family history of preterm labor, she is at significant risk for preterm birth of her baby. So, it is essential to anticipate preterm labor and give appropriate counselling to the mother regarding the factors which may contribute to preterm labor and regarding preventable etiological factors and to conduct frequent antenatal checkups. Thus, reducing the personal expenditure and government expenditure in government hospitals by preventing preterm births.

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  Table 1
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  Table 2