American Academy of Pediatrics

TABLE 1

Statistical and Conventional Definitions of Weeks of Gestation, Completed Weeks of Gestation, Late Preterm Gestation, and Term Gestation

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Late-preterm gestation is defined by medical convention as 34

\({0}/{7}\)

weeks (239 days) through and including 36

\({6}/{7}\)

weeks (259 days) after the beginning of the mother's last normal menstrual period. This is indicated in days with a red background. For comparison, term gestation spans from 37

\({0}/{7}\)

weeks (260 days) through and including 41

\({6}/{7}\)

weeks (294 days) after the beginning of the mother's last normal menstrual period, which is indicated in days with an aqua background.

Completed week of gestation indicates the number of 7-day intervals that have passed since the beginning of the mother's last normal menstrual period. For example, the first completed week occurs after 1 seven-day interval (0

\({6}/{7}\)

th week or 7 days) has passed. The 37th completed week occurs after 37 seven-day intervals (36

\({0}/{7}\)

weeks or 259 days) have passed.

Fraction of a week indicates the days of each gestational week as a fraction. For example, the first day of gestation is the first day of the mother's 36

\({0}/{7}\)

th week of gestation and ends on the 36

\({6}/{7}\)

th week of gestation.

Statistical day indicates that the first day of the mother's last menstrual period begins as day 0 and is not complete until the beginning of day 1.

This statistical view of gestational age differs by 1 day from the conventional medical count of days, which indicates that the first day of the mother's last menstrual period begins as day 1. This important difference is indicated by the statistically defined days that have a gray background and conventionally defined days having no background or a red or aqua background.

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