(CNN)New information on how having a sexually transmitted infection before or during pregnancy can affect newborns is emerging.
Having chlamydia, gonorrhea or syphilis -- sometimes called "silent" infections due to a dearth of apparent symptoms in the early stages -- has been associated with a higher risk of giving birth too early, according to a study published Monday in the journal JAMA Network Open.
Preterm birth -- when a baby is born before 37 weeks' gestation -- is the leading cause of newborn complications and death worldwide, affecting 10% of live-born deliveries in the United States, the study authors wrote. Also, the number of preterm births slightly rose from 2016 to 2019, so more work to identify risk factors has been needed.
US cases of chlamydia, gonorrhea and syphilis also increased from 2013 to 2018, according to the US Centers for Disease Control and Prevention. Maternal STIs have been known to harm developing fetuses, but previous studies on this link have been inconclusive, the researchers wrote -- so the authors used nationwide birth certificate data and medical records of more than 14 million mother-infant pairs to examine the links between preterm birth and mothers' infections.
Of all US mothers who had given birth between January 2016 and December 2019, 1.9% (267,260) had chlamydia, 0.3% had gonorrhea and 0.1% were infected with syphilis before or during their pregnancies. Among all newborns, 8% (1,146,800) were born prematurely. Mothers with chlamydia, gonorrhea or syphilis were 1.04, 1.10 and 1.17 times more likely to have a moderately preterm birth -- meaning between 32 and 36 weeks of gestation -- than women without STIs, respectively, the authors found.
Infections with gonorrhea and syphilis were more strongly associated with very preterm births, which meant between 28 and 31 weeks.
"Even though some of the effects are small, STIs do appear to increase your chance of delivering preterm," said Kelli Ryckman, a coauthor of the study and a professor of epidemiology at the University of Iowa's College of Public Health.
How STIs could affect birth time
Chlamydia and gonorrhea are common sexually transmitted infections that can affect people of all genders who have vaginal, anal or oral sex with someone who is infected, according to the CDC. Chlamydia can cause serious, permanent damage to a woman's reproductive system. And both STIs can cause preterm birth and be transferred to babies during childbirth, possibly causing pneumonia and ear infections.