Concept of proximate determinants (cont.)
Reproductive span of a woman (cont.)
The duration of birth intervals is determined by the following three factors in the absence of intrauterine mortality:
1. Postpartum infecundable period
Immediately following a birth, a woman experiences an infecundable period during which the regular pattern of ovulations is absent. Women do not have regular menstruation during this period, and this absence of menstrual period is called postpartum amenorrhea. The duration of postpartum amenorrhea is associated with frequency and duration of breastfeeding. Also in some societies, especially in sub-Saharan Africa, postpartum abstinence is prevalent. The combined effect of postpartum amenorrhea and abstinence is termed postpartum infecundability or postpartum insusceptibility.
2. Waiting time to conception
This period refers to the time between resumption of ovulation and conception. Return to regular menstruation is usually regarded as resumption of ovulation. The length is inversely associated with the use and effectiveness of contraception and fecundability, which is largely determined by the frequency of sexual intercourse.
3. A full-term pregnancy
Since the duration of pregnancies ending in live birth varies little, it is convenient to assume that this length is a constant duration of 9 months.
Spontaneous or induced intrauterine death
In case a pregnancy ends prematurely in a spontaneous or induced intrauterine death, the birth interval extends due to a shortened pregnancy, a brief infecundable period and a further waiting time to conception.
In populations where parity-specific fertility limitation is rare, married and fecund women reproduce at a rate inversely related to the average duration of birth interval. Short birth intervals are related to high level of fertility. Therefore, it is important to understand how length of birth intervals is determined especially in populations with low contraceptive prevalence.
By reviewing reproductive life span of women, Bongaarts identified that the following seven proximate determinants may vary across populations and over time. They are: marriage or in sexual union; onset of permanent sterility; postpartum infecundability; fecundability or frequency of sexual intercourse; use and effectiveness of contraception; spontaneous intrauterine mortality; and induced abortion. He further created a framework of the proximate determinants of fertility, which will be further discussed in the next pages.