Estimation of fecundity and secondary sterility from survey data on birth intervals in Egypt
El-Shalakani, M.H.; Suchindran, C.M.
Human Biology 65(1): 59-70
1993
ISSN/ISBN: 0018-7143 PMID: 8436391 DOI: 10.2307/41464362Document Number: 323771
Fecundity is estimated from last closed birth interval (LCBI) data using a steady state model. An inflated model of theopen birth interval (OPI) is derived on the assumption that the birth interval from first birth is marked by a certain proportion of women who become secondarily sterile or choose to be so to avoid further births. Biases such as truncation, censoring, and selectivity are overcome by using birth intervals and a stochastic model. Data were obtained from the 1980 Egyptian Fertility Survey to estimate fertility and secondary sterility of noncontracepting, currently married women by age, residence,and educational status. The model is specified, the data identified, and the process of application presented. The gestation period is assumed to be 9 months, and postpartum amenorrhea (PPA) constructed by age, parity, and duration of marriage. The constant period of nonsusceptibility is considered to be the gestation period of 9 months plus the average value of PPA by socioeconomic group. In the inflated model of OBI, a truncated model of OBI is constructed at different points designated as the risk of occurrences (lambda) and the probability of a birth after the first birth (alpha), which are obtained by the Gauss-Newton and Marquardt iterative methods. The results show the expected: that fecundity decreases with age. After 25 years of age, the fecundity parameter lambda shows a steady and diminishing decline as age increases. There is some evidence of declines in fecundity from rural to urban areas. No schooling to 3 years plus of schooling shows a similar pattern. The patterns of women aged 30 years reflects the opposite education effect, which might be interpreted as high coital frequency in modern marriages and among more educated women. The inflated model of OBI follows similar trends, but OBI data have higher estimates of fecundity for younger women and lower estimates for older women than LCBI data. Age patterns of secondary sterility increase until age 30; the proportion of sterile women increases thereafter.