May 15 (UPI) -- The number of stillbirths and newborn deaths in the United States declined 11.5 percent between 2007 and 2015, according to a study of nearly 35 million births.
The study examined trends in U.S. perinatal mortality by linking live birth and infant death data among 34,236,577 live births and 199,838 stillbirths. The findings by leading author Cande Ananth, a professor of obstetrics and gynecology at Columbia University Medical Center, were published Monday in the Journal of American Medical Association Pediatrics.
The study looked at data from the National Center for Health Statistics of the U.S. Centers for Disease Control and Prevention -- which covers 99 percent of the births.
Stillbirths are defined as death of a fetus at 20 or more weeks of gestation and neonatal death is within 28 days of delivery, according to the CDC.
The stillbirths and neonatal deaths decreased from 37,813 deaths in 2007 to 33,457 in 2015.
Stillbirths declined from 5.7 to 5.6 per 1,000 births and neonatal mortality from 3.3 to 3.0 per 1,000 births.
The CDC earlier estimated about 1 percent of all pregnancies end as stillborn in the United States, which is about the same rate that die in the first year.
The proportion of births to women younger than 20 years old decreased from 10.5 percent in 2007 to 5.9 percent in 2015. Births to women aged 35 to 39 years increased from 11.4 percent to 13.1 and births to women 40 years or older went from 2.5 percent to 3.0 percent.
The study also looked at changes in the average gestational age at birth between 2007 and 2015. The age of the embryo or fetus is typically calculated beginning at the first day of the mother's last menstrual period.
The groups were classified into eight stages: extremely preterm, 20-27 weeks; very preterm, 28-31 weeks; moderately preterm, 32-33 weeks; late preterm, 34-36 weeks; early term, 37-38 weeks; full term, 39-40 weeks; late term, 41 weeks; or post-term, 42-44 weeks).
Birth rates decreased in all groups except one the 39- to 40-week group, which rose from 54.5 percent in 2007 to 60.2 percent 2015.
"Athough the proportion of births at gestational age 39 to 40 weeks increased, perinatal mortality at this gestational age declined," the researchers wrote. "This finding may be owing to pregnancies delivered at 39 to 40 weeks that previously would have been unnecessarily delivered earlier, leaving fetuses at higher risk for mortality at other gestational ages."
The proportion of births at 20 to 27 weeks decreased by 8.1 percent, but perinatal mortality rates increased 0.8 percent and neonatal mortality rates were up 0.9 percent annually at this gestational age.
In 2013, the American College of Obstetrics and Gynecologists recommended that obstetricians postpone elective deliveries until 39 weeks to avoid complications of preterm delivery such as problems with breathing, heart problems, brain, gastrointestinal, metabolism, blood, according to the Mayo Clinic.
Risk factors include a mother's history of preterm birth; problems of the uterus, cervix or placenta; smoking or illicit drugs use during pregnancy; high blood pressure and diabetes; infections; overweight and underweight; stressful life events; physical injury or trauma, according to the Mayo Clinic.
"But premature birth can happen to anyone. In fact, many women who have a premature birth have no known risk factors," Mayo Clinic wrote.