Conventional wisdom always held that pregnancy is riskier for women over the age of 35. New research, published in the medical journal Obstetrics and Gynecology, suggests age-related risks actually begin as a woman leaves her 20s. The researchers note that women in Western cultures tend to have children later in life, increasing the risk of complications such as preterm birth, slow growth, and stillbirth.

Researchers analyzed health records gathered from 955,804 first-time mothers who delivered babies in Sweden and Norway between the years of 1990 and 2010. The scientists separated the women into four age groups: 25 to 29 years old; 30 to 34 years old; 35 to 39 years old; and 40 years and older.

The scientists decided to use an odds ratio system to assess the risk for pregnancy later in life. An odds ratio (OR), measures the association between an event and an outcome. An OR expresses the odds that the event will cause a specific outcome. When something has an odds ratio of exactly one, it means the event is not likely to affect the outcome. An OR greater than one means the event increased the odds of a specific outcome; the larger the number, the more likely the event will cause the expected outcome.

In this study, the researchers used the information gathered from the health records to calculate odds ratio that the study participants would have complications such as premature birth, babies who are small for their gestational age, stillbirths, poor tolerance to the birthing process and infant death.

The scientists found that first-time mothers aged 30 to 34 had a greater risk for premature birth or stillbirth than did women between the ages of 25 and 29 years. While the risk for individual women

Other factors, such as obesity and smoking, also increased the odds of complications. The researchers urged women who hoped to become pregnant someday to avoid smoking and obesity. The scientists also say they will soon use information gathered from a database registry of 2.2 million women to assess the potential risk of giving birth to a second or third child during these later years. They found the OR for 30 to 34-year-old Swedish women was 1.24, meaning there was a strong association between advancing age and pregnancy complications. OR among women from Norway was even higher at 1.26.

Source: Waldenström, Ulla, Ph.D., and Vigdis Aasheim, MSc. "Adverse Pregnancy Outcomes Related to Advanced Maternal Age Compared with Smoking and Being Overweight." Obstetrics & Gynecology. N.p., 6 Dec. 2013. Web. 20 Dec. 2013.