Older Nulliparous Women Have Higher Risk for Stillbirth

Arielle Emmett

July 11, 2015

A large, Swedish, population registry study suggests that nulliparous women 35 years of age or older are at greater risk for stillbirth compared with both younger women and similarly aged parous women.

Although previous studies have identified that advanced maternal age and first-time birth status are both independent risk factors in stillbirths, this cohort study of more than 1.8 million pregnancies is one of the few to examine advanced maternal age and parity status as a combined factor in stillbirth risks among women.

The researchers found that risk for stillbirth increases among older mothers, especially those giving birth for the first time at ages 35 years or older.

However, age-related risk was reduced or eliminated among highly educated women who have already given birth to one or more children. Physiological changes during first pregnancy may have a protective effect, the authors suggest, with increased blood flow to the uterus, while countering the effects of placental aging.

The study was published in the August issue of Obstetrics & Gynecology.

Ulla Waldenström, PhD, from the Karolinska Institutet, Stockholm, Sweden, and colleagues analyzed data from the Swedish Medical Birth Register between 1990 and 2011 to estimate risk for stillbirth among women in four different parity groups at ages 30 to 34 years, 35 to 39 years, and 40 years and older compared with women aged 25 to 29 years. The authors adjusted for such factors as levels of education, smoking, body mass index, maternal diseases, history of stillbirths, and intervals between deliveries.

Stillbirth rates increased with advancing maternal age, going from 0.27% at ages 25 to 29 years to 0.31% at ages 31 to 34 years, to 0.40% at ages 35 to 39 years, and to 0.53% in women aged 40 years or older.

When the authors restricted their analysis to first-time mothers, the increase in risk was even greater, rising from 0.34% in the youngest women to 0.44% among women aged 30 to 34 years, 0.57% among women aged 35 to 39 years, and 0.71% among women aged 40 years and older. When adjusted for health and demographic factors, the odds ratio for a nulliparous women was 1.50 (95% confidence interval, 1.05 - 2.15) compared with for the youngest group.

However, when the researchers analyzed two low-risk groups, including women with a college or university education and those of normal weight who do not smoke, they discovered that the age-associated risk appeared to be mitigated. In addition, adjusting for maternal morbidity and small for gestational age also appeared to reduce the risk for stillbirth among older women.

The authors conclude that age-related risk is reduced or eliminated in parous women, but that advanced maternal age still carries the highest risk for a stillbirth for women attempting first-time births.

The authors have disclosed no relevant financial relationships.

Obstet Gynecol. 2015;126:355-362.

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