(CNN)If you're stressed or wanting to enjoy virtual happy hour with friends while pregnant, having a glass of wine every so often may seem like a relaxing plan.
But that behavior runs counter to a study published Tuesday in the journal JAMA Network Open.
Researchers found drinking or smoking of any level while pregnant — from low to high, and even if you quit early — influenced the brain development of the mothers' newborns.
Negative long-term effects of excessive prenatal alcohol or tobacco exposure (or both), increase the risk for multiple adverse outcomes, researchers observed.
Prenatal alcohol exposure is the leading cause of preventable intellectual disability, the study said, and smoking during pregnancy is one of the most modifiable causes of post-birth disease and death.
However, public health hasn't yet determined a safe level of alcohol or tobacco use during pregnancy, the study said. So the authors examined the link between prenatal exposure to alcohol and tobacco smoking and brain activity in newborns.
They didn't find a safe, conclusive level, but the findings suggested drinking or smoking in early stages and then quitting, or doing so at low, moderate or high levels all impacted newborn brain development.
Those who drank alcohol then quit in the first trimester had an average of nine drinks during that time period. A low level of alcohol is defined for the study's purposes as two standard drinks on average during the first trimester.
A moderate or high level meant, on average, 36 drinks during the first trimester, 16 in the second trimester and eight in the third trimester. (The authors didn't have enough participants to split between moderate and high level groups, so they combined them.)
For smokers, 14 cigarettes on average in each trimester was considered "low" and 45 to 50 cigarettes on average was deemed high. Those who quit smoking early smoked an average of seven cigarettes before quitting.
"Already we can see there was an alteration in the development of brain processes that can be quantified during sleep when babies are just a few days of age," said senior author William P. Fifer, a professor of medical psychology at Columbia University Irving Medical Center in New York City. "Babies of those moms who even had low levels of alcohol or tobacco exposure still had some alterations in the brain activity."
Effects of alcohol and tobacco on brain activity
Participants were 1,739 newborns enrolled in the Safe Passage Study, a large multidisciplinary study investigating the association between prenatal alcohol exposure, sudden infant death syndrome and stillbirth. The Safe Passage Study also determined the outcomes of prenatal exposures and how they're affected by environmental and lifestyle factors. The Safe Passage Study has concluded, but the current article is a follow up of the newborns included.
Their mothers were recruited from 2011 through 2015 from clinical sites in Cape Town, South Africa, and the Northern Plains region of the United States. Babies who were born at less than 37 weeks or more than 41 weeks gestation and mothers who birthed more than one newborn were excluded. Mothers who took psychiatric medication were also excluded from the current study but included in the previous Safe Passage Study.