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Reported as age at last birthday (that is, age in completed years), often calculated by subtracting a person’s date of birth from the reference date, with the reference date being the date of examination, interview, or other contact.

Mother’s (maternal) age is reported on the birth certificate by all states. Birth statistics are presented for mothers ages 10–49 through 1996 and ages 10–54 starting in 1997, based on mother’s date of birth or age as reported on the birth certificate. The age of the mother is edited for upper and lower limits. When the age of the mother is computed to be younger than 10 or 55 and older (50 and older in 1964–1996), it is considered not stated and is imputed according to the age of the mother from the previous birth record of the same race and total birth order (total of fetal deaths and live births). Before 1963, not-stated ages were distributed in proportion to the known ages for each racial group.

Beginning with 2003 data, age of mother was imputed for stated ages younger than 8 and 65 and older for births reported using the 2003 revision of the birth certificate. Starting with 2007 data, age of mother is imputed for all births for stated ages younger than 8 and 65 and older, regardless of the birth certificate version used. As with data for earlier years, age is imputed according to the age of mother from the previous record with the same race and total birth order.

Before 1997, the birth rate for maternal age group 45–54 included births to women ages 45–49 in the numerator and was based on the population of women ages 45–49 in the denominator (that is, women ages 49–54 were not included in the numerator or denominator). From 1997 through 2006, the birth rate for maternal age group 45–54 was expanded to include births to women ages 45–54 in the numerator and was based on the population of women ages 45–49 in the denominator. Starting in 2007, the birth rate for maternal age group 45–54 is expanded to include births to women age 45 and older in the numerator and is based on the population of women ages 45–49 in the denominator.