Pregnancy outcome and the time required for next conception, Jain (1969) looks at a large group of presumably fertile women and seeks to determine the average age of conception after a previous pregnancy or the length of time between pregnancies in the absence of birth control. According to the study the average length of time between pregnancies remains high in those under the age of 20, then levels out to spike again in individual women according to advancing age. Additionally the outcome of the pregnancy was also considered, where those who had live births ending the pregnancy or fetal death via natural or artificial means also experienced shortened pregnancy intervals, though higher among ages less than 20 and which gradually increases with maternal age. Hypotehsis

The paper hypothesis surrounds an empirical exploration to add to the knowledge base regarding the length of pregnancy interval by age of women and to help provide an inferred length of amenorrhea among these same women all in the absence of contraception, with the assumption that in the absence of contraception the natural pregnancy interval will be determined.

Statistical Methodology

The methodology employed is demonstrative of a simple mean and variance report, where the average was determined and then a variance among all participants was determined. The mean is determined in this study by adding all the interval lengths in months of all the participants and then dividing by the number of participants to determine the average between them. The mean is then calculated based on both maternal age as well as the outcome of the pregnancy i.e. live birth, termination of pregnancy and infant death at birth, prior to one year of age and after one year of age. The variance is determined by finding the square of the standard deviation, which is the square root of the variance. The sample is derived from a 2,443 sample of surveyed individual married women who recorded their pregnancy histories prior to beginning a family planning program, in Taichung (Taiwan) known as the Intensive Fertility Survey (1962). The sample are then weeded out by eliminating the data associated with those who reported any contraception use (only about 7%). (422) Additionally the researchers then further reduce the statistical analysis data by eliminating the first pregnancy interval or the time it took from marriage to first pregnancy end, as well as the last (open)pregnancy interval the last pregnancy end to the survey time so as not to skew the data unnecessarily. The work also notes two statistical biases that are significant for obtaining accurate data and then attempts to account for them statistically, a truncation bias where the researchers believed that the collective data of all women irrespective of age would bias the results, to account for this possible bias the researchers offered statistical analysis of two methods of data analysis. One where the interval determination was the same for all participants and one where the age was measured at the beginning of pregnancy intervals for age groups less than 20 and 20-24 and at the end of pregnancy intervals for the age groups 25-29,30-34 and 35-39. (424-425) The second potential bias discussed is a memory bias, associated with the inability of individuals to remember pregnancy...

The researchers go on to state that this number was greatly affected by the outcome of the preceding pregnancy, where pregnancies that resulted in live birth and an infant that survived to one year the interval was 17 months. Additionally, if the preceding pregnancy ended in fetal death prior to one year the following pregnancy interval was shortened by 6 months. The work then goes on to statistically compare the two methods of age classification and the statistical comparison between the various ages, which included a high pregnancy interval in the less than 20 age group that then declined again in the 20-24 age group and gradually increased in the remaining age groups 25-29,30-34 and 35-39. (424-430) Finally, the researcher then attempts to conclude the statistical data on the length of amenorrhea following pregnancy termination. (430-433)
Conclusion

This work is clearly dated, (1969) but it is also important to note that the age of the dataset may be essential for the data collected as finding a large population of married women who practice no contraception is exceedingly rare given the pervasive nature of contraception use all over the world today. A much more timely work associated with pregnancy interval, looking specifically at the impact of pregnancy interval (intentional or otherwise) and the outcome of pregnancies (i.e. fetal prematurity, underweight or death) utilized a much larger sample of 14,930 women. The work did not use multiple pregnancy data (eliminating memory bias) but instead used the documented pregnancy interval from the termination of the previous pregnancy to the beginning of the next pregnancy (based on due date calculations) after termination of current pregnancy. (Cecatti, Correa-Silva, Milanez, Morais, & Souza, 2008) The data collection for the Jain study clearly could have used this method of collection (i.e. current to past for a current pregnancy based on report or medical records) to arrive at a dataset with less unknown and/or notable memory bias. Though the intention of the Jain work was to inflate data availability by including multiple pregnancy via survey self-report the inclusion of memory bias as well as self-report bias using this method is questionable. The intention of the second work additionally was to determine the outcome of pregnancies, i.e. maternal and infant health based on pregnancy intervals as opposed to just determining the interval itself which is reported by the researcher to be a median of 27 months, which again varied by age and other demographic factors. The findings of this research article are also based upon only pregnancies that end in live birth in the current pregnancy, and again are based on the end of the interval between the termination of the last pregnancy and the end of the current pregnancy, minus the gestation period. The mean pregnancy interval was not calculated as this was not of interest to the researchers but the largest statistical group (34.6%) included pregnancy intervals of