Inter-pregnancy interval (IPI) is defined as the interlude between delivery of the earlier infant and conception of the existing pregnancy.
This definition excludes miscarriage as a preceding pregnancy event. Often the IPI is calculated as the interval between live births, ignoring miscarriages or fetal deaths in between them. The interval is estimated as the difference between the date of delivery of the subsequent pregnancy and that of the previous pregnancy minus the gestational age of the subsequent pregnancy. Gestational age is estimated as the interval in completed weeks from the last normal menstrual period (LNMP) to the child’s date of birth.