(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.
This study didn't investigate exactly how having STIs before or during pregnancy could affect when infants are born, but the authors have some ideas. Chlamydia and gonorrhea could move up through the vagina and cervix and influence the occurrence of chorioamnionitis, a condition wherein bacteria infects the membranes surrounding the fetus and the fluid in which the fetus floats.
Syphilis -- spread by direct contact with a syphilis sore on or near the penis, vagina, anus, rectum or mouth -- may cause systemic infection and trigger the placenta to use its inflammatory response, the researchers suggested.
"All of these responses may cause inflammation and activate the maternal and/or fetal immune system, which is an established cause of preterm birth," the researchers suggested. "However, whether a maternal infection induces preterm birth may also depend on the characteristic and concentration of the pathogen and the timing of infection."
The "massive sample size of this study" and other aspects might suggest precision, but "do not necessarily suggest validity," wrote Drs. Emily Adhikari and Scott Roberts in a commentary on the study. The doctors, who are affiliated with the obstetrics and gynecology department at the University of Texas Southwestern Medical Center, Dallas, weren't involved in the study.
Observational studies such as this one can't take into account individual behaviors and other factors potentially responsible for preterm birth, including chronic stress, gestational age of diagnosis or lack of access to treatment, Adhikari and Roberts wrote.
"Without a better understanding of which infections were and were not treated, we have little to guide a search for targeted interventions to prevent preterm birth in a cohort with treatable infectious diseases," they added.
Being in a long-term, mutually monogamous relationship with someone who has recently been tested for STIs and received negative results can lower your risk of contracting an STI, the CDC says. You can also protect yourself by using latex condoms correctly every time you have sex. However, know that you can still get syphilis from sores in areas not covered by a condom.
For women, symptoms of chlamydia, gonorrhea and syphilis don't always appear given the sometimes silent nature of early stages of infection. But symptoms can include abnormal vaginal discharge or bleeding; burning sensation while peeing; rectal pain, discharge, itching or bleeding; or painful bowel movements. Early signs of syphilis are sores that can be firm, round and painless.