Women aged 40 and older have greatest likelihood of stillbirth; teenagers also have an elevated risk.

Teenagers and women aged 35 or older are at significantly higher risk of having a stillbirth than are women in their 20s and early 30s, and the association remains even once a large number of known risk factors for stillbirth are taken into account, according to a study based on nearly six million deliveries that occurred nationwide between i995 and 2002. (1) The greatest risk of stillbirth is among women aged 40 or older; the odds of stillbirth in this age-group are almost twice those among women aged 20-34.

Because most studies of the relationship between maternal age and stillbirth have been conducted in hospitals or among homogeneous populations, researchers revisited the issue using a database that includes about 20% of all patients admitted to nonfederal U.S. hospitals and represents a diverse population of women giving birth. The database contains information on about 5.9 million women who were hospitalized for a delivery during the study period, of whom 12% were 19 or younger, 75% were aged 20-34, 11% were aged 35-39 and 2% were 40 or older. (Virtually all were 12-47 years old.) The researchers examined data on these women's risk factors for stillbirth, calculated rates of stillbirth and used multiple logistic regression to assess the relationships between maternal age and other characteristics and the likelihood of stillbirth.

The racial and ethnic makeup of the study population and the prevalence of known risk factors for stillbirth differed by age-group. White women and Asians and Pacific Islanders were overrepresented among mothers aged 35 or older, and blacks and Hispanics were overrepresented among those younger than 35. The proportion of women who had multiple gestations increased steadily from less than 1% among teenagers to nearly 3% among women aged 40 or older. Hypertension was most common among the youngest and oldest mothers (affecting 8% and 11%, respectively), and rates of tobacco dependence and infection of the amniotic cavity declined with age (from 2-3% among teenagers to less than 2% among women aged 40 or older). One percent of teenagers had diabetes, but the rate rose rapidly with age, to 10% among the oldest mothers. Whereas the proportion of women experiencing placenta previa (implantation of the placenta close to or over the opening to the birth canal) increased steadily with age, the proportion experiencing placental abruption (separation of the placenta from the uterus) was relatively high at the extreme ages but dropped among women in their 20s and early 30s; a similar pattern was found in rates of fetal abnormality.

Stillbirths accounted for about 15 of every 1,000 deliveries for mothers aged 12-13; the rate dropped precipitously, to nine per 1,000, by age 15 and remained below that level until the mid-30s. By age 37, it again exceeded nine per 1,000, and after a steady climb, it reached nearly 22 per 1,000 among 47-year-olds. The pattern was the same, although rates were lower, in analyses excluding multiple gestations. Furthermore, the incidence of stillbirth changed little over the study period, and similar disparities by maternal age were apparent in each year.

Results of the multivariate analysis indicated that teenagers and women aged 35 or older were significantly more likely than 20-34-year-olds to have a stillbirth (odds ratios, 1.1 for women aged 19 or younger, 1.3 for those in their late 30s and 1.7 for those aged 40 or older). Asians and Pacific Islanders had lower odds of stillbirth than white women (0.9), but the likelihood of this outcome was elevated among other nonwhite women (2.1 for blacks, 1.2 for Hispanics and 1.5 for Native Americans). The odds of stillbirth were elevated for women who had multiple gestations (6.2) and for those who had various diseases or substance dependence-for example, hypertension (1.1), tobacco dependence (1.1), other substance use or dependence (1.7) and infection of the amniotic cavity (5.1). Diabetes, a known risk factor for stillbirth, was not associated with its occurrence in these analyses. Several placental problems predicted an increased likelihood of stillbirth; notably, the odds were eight times as high among women with placental abruption as among those who did not have this condition. Fetal abnormality was associated with a dramatic increase in the odds of stillbirth (19.0).

Despite the advantages of the database, the researchers note that it also imposed certain limitations on the study. For example, the definition of important concepts, such as gestational age, was not standardized, and some potential risk factors were not coded and therefore could not be included in the analyses. However, the researchers comment that their finding of an increased risk of stillbirth among mothers at the extremes of the age spectrum, and the persistence of this association in multivariate analysis, suggests that the mechanism underlying the link between maternal age and stillbirth requires further exploration "so that appropriate interventions and improved outcomes can be obtained."

REFERENCE

(1.) Bateman BT and Simpson LL, Higher rate of stillbirth at the extremes of reproductive age: a large nationwide sample of deliveries in the United States, American Journal of Obstetrics and Gynecology, 2006, 194(3): 840-845.