Postpartum infecundability (postpartum insusceptibility)
Index of postpartum infecundability (Ci)
The index of postpartum infecundability is intended to measure the effects on fertility of extended periods of postpartum infecundability, or often called postpartum insusceptibility, due to postpartum amenorrhea and postpartum abstinence (Bongaarts 1982). The index is calculated as:
Ci = 20 / (18.5+i)
where
i = average duration of postpartum infecundability (postpartum insusceptibility) caused by breastfeeding and postpartum abstinence.
If no breastfeeding or postpartum abstinence is practised, the birth interval averages about 20 months. This is made up of:
- 1.5 months minimum postpartum infecundable period
- 7.5 months of waiting time to conception on average
- 2 months of time added by spontaneous intrauterine mortality on average
- 9 months for a full-term pregnancy
Therefore, in the presence of breastfeeding and postpartum abstinence, the average birth interval equals, approximately, 18.5 months (7.5+2+9) plus the duration of postpartum infecundability, which is defined by length of postpartum amenorrhea and abstinence. This factor therefore measures the fertility-inhibiting effect by taking the ratio of the minimum birth interval (20 months) and that extended by postpartum amenorrhea or abstinence.
Figure 6: Reproductive life span of women
