A premature birth takes place more than three weeks before the baby would have been due under normal circumstances, i.e. before the beginning of week 37 of the pregnancy. A premature baby has less time to develop before it is born.
Two current tests that screen for preterm birth risk rarely spot trouble in first-time pregnancies, a new study suggests.
The tests' predictive powers were assessed for naturally occurring preterm deliveries only, and not for medical procedures such as Caesarean surgery or induced labour.
The study was published in the Journal of the American Medical Association.
"These methods of assessing women in their first pregnancy do not identify most of those who will later go on to have a spontaneous preterm delivery," said senior study author Dr Uma Reddy. She is with the Pregnancy and Perinatology Branch at the US National Institute of Child Health and Human Development.
Findings from a previous University of Rhode Island study revealed that preemies grow up to be less healthy, struggle more socially and face a greater risk of heart problems compared to those born full-term.
Need for better screening
"There is a need to develop better screening tests that can be performed early in pregnancy," Reddy said in an institute news release.
Preterm birth is the leading cause of neonatal death or long-term disability, according to the US Centers for Disease Control and Prevention.
In the United States, more than one in 10 births were preterm in 2015, and about 35% of infant deaths were related to prematurity, the agency said.
Two types of screening tests
Two screening tests are typically used to try to predict which women are at high risk for preterm birth, which is hard to predict until it begins, researchers from the University of Utah said in a university news release.
One of the tests is an ultrasound of the cervix. Previous research has suggested that a short cervix early in pregnancy could be a warning sign of preterm birth. The second test checks for foetal fibronectin, a glue-like protein that secures the amniotic sac to the uterus. Some studies have suggested that the presence of foetal fibronectin in the vagina early in pregnancy could also signal an increased risk of preterm birth risk.
To gauge the effectiveness of each of those tests – or combining both of them – in predicting preterm birth, researchers at eight clinical centres across the United States tested thousands of women at three points during their pregnancies. The aim: to see whether the results predicted which women would deliver prematurely.
The tests were conducted at around 12 weeks, 19 weeks and 28 weeks of pregnancy. The study didn't include women who'd miscarried before 20 weeks or who had terminated a previous pregnancy, the researchers noted.
Tests ineffective to spot preterm birth
After screening more than 9 000 women with first-time pregnancies, the researchers found the two tests identified only a small number of women who would eventually deliver their baby prematurely.
Study lead author Dr Sean Esplin is a maternal-foetal medicine specialist at Intermountain Medical Center in Salt Lake City and an obstetrics-gynaecology professor at the University of Utah School of Medicine.
"This is a huge trial. This study was our best hope to say how effectively these tests predict the likelihood of preterm birth, and they weren't as effective as we'd hoped," Esplin said.