Around 1 in 8 babies in the United States is born premature, increasing the risk for long hospital stays and health problems down the road.
But knowing which first-time moms are more likely to go into labor preterm is not easy.
"The largest risk factor for preterm birth is a history of preterm birth, which by definition, first-time mothers don't have," said Dr. Erin Clark, an assistant professor of obstetrics and gynecology at the University of Utah Health Sciences Center. "Predicting which first-time mothers will deliver early is very difficult."
A large, new study out of Australia attempted to clarify the issue, pinpointing several factors that seem to increase a woman's risk for early delivery. Those factors include an overall lack of feeling well, a short cervix and marijuana use before pregnancy.
In findings published in the online journal PLoS One, researchers considered data from more than 3,000 first-time moms in New Zealand and Australia. Around 5 percent of the new mothers gave birth before 37 weeks, or at least three weeks early.
Having a mother or sister who gave birth to a baby with a low birth weight was among the biggest risk factors for preterm birth, as was a maternal family history of diabetes or preeclampsia, which occurs when women develop high blood pressure and proteins in the urine during pregnancy.
Other risk factors were shorter cervical length and regular marijuana use prior to pregnancy.
The latter, according to Dr. Michael Katz, interim medical director for the March of Dimes, should not unduly frighten women who may have used marijuana in the past.
"I would not attribute much importance to that other than, 'Ok, it's an observation','' Katz told The Huffington Post. "Certainly, if you were a woman trying to become pregnant and you asked me 'Should I use marijuana?' My answer would be 'no'."
What is not clear, said Katz, is whether there is some mechanism underlying the potential link between marijuana use and preterm birth, or if women who smoke are more inclined to engage in generally unhealthy behaviors.
While experts praised the study's thoroughness, they said it does very little to change the understanding of risk factors in first-time moms. Rather, it reinforces that there are many things before and during pregnancy that can influence risk.
"Our ability to predict preterm birth remains sub-optimal," Clark said. "Even with a detailed history and physical exam, we remain largely unable to tell pregnant women when their baby will deliver. And because we can't tell who will deliver early, we have little opportunity to try to intervene before labor starts."
Most pregnancies last 40 weeks, and babies born between 37 and 42 weeks are considered full-term. Babies born before 37 weeks are generally considered premature, though the earlier the baby is born, the more severe his or her health issues may be. According to health organizations such as the Centers for Disease Control and Prevention, women can reduce their risk of preterm birth by avoiding drugs and alcohol, receiving prenatal care and monitoring their blood pressure.
Most women who deliver preterm have no known risk factors, the CDC states. And that, according to Clark, is "why the race for a predictive biomarker is on."
"Ultimately, we need far more effective prediction and intervention strategies if we are going to impact the rate of preterm birth," she said.