Delayed Childbearing and Its Impact on Population Rate Changes in Lower Birth Weight, Multiple Birth, and Preterm Delivery

Abstract

Women are increasingly delaying childbearing until their fourth or fifth decade, posing the question of how this affects birth outcomes. The authors attempted to relate delayed childbearing at age 35 years and older to population rate changes in low birth weight (LBW), preterm delivery, and multiple births in Alberta, Canada in the years 1990 to 1996. The criterion for LBW infants was 2,500 gm, and for preterm delivery, 37 weeks' gestation. Small-for-gestational-age (SGA) infants were less than the 10th percentile in birth weight. Infants born in Alberta during the study years totalled 283,956. The proportion of infants born to mothers aged 35 years and older increased by more than half, from 8.4% in 1990 to 12.6% in 1996. Compared with the general provincial population, infants born to women aged 35 and older were significantly likelier to be LBW infants, to be born before 37 weeks' completed gestation, to be part of multiple births, and to be stillborn (Table 1). In most weight and gestational-age subgroups, delayed childbearing had a substantial effect on the increase in LBW and preterm births. The twin rate among women aged 35 and older increased 29% between 1990 and 1996, although the population rate of twin births rose by 15%. Women older than 35 years accounted for 68.5% of the increase in triplets in the province over these years. Maternal age did not influence SGA singleton births during the study period, whether preterm or term. These findings suggest that recent increases in LBW infants and preterm delivery are in part related to delayed childbearing. The fact that maternal age was unrelated to changes in SGA suggests that the age effect acts through pregnancy complications that result in preterm delivery and LBW infants. Prospective parents should be made aware of the increase neonatal morbidity risk associated with childbearing after age 35 years.