出典:国際連合
Studies of birth timing 1 deal with the length of birth intervals 2. These include the interval between marriage and the first birth 3 and intervals between successive births 4. The interval between a birth and a fixed date, such as that of a census (202-1 *) or survey (203-4), is called an open birth interval 5; intervals that begin before and end after that date are called straddling intervals 6. The interval between marriage and the Nth birth 7 is also used to study the timing of births.