Breastfeeding mothers who drink alcohol may be more likely to have children with cognitive problems than women who abstain while nursing, an Australian study suggests.

For the study, researchers examined results of reasoning tests completed by 5,107 children, as well as questionnaires completed by their mothers detailing whether they nursed babies at all and how often they smoked cigarettes or drank alcohol while pregnant or breastfeeding. Breastfed babies had lower nonverbal reasoning scores at six to seven years old when their mothers consumed any alcohol while during the period they were nursing, and kids’ scores looked worse the more women drank, researchers report in Pediatrics.

“This matters because it suggests that there is no safe level of alcohol for a breastfeeding mother to drink,” said lead study author Louisa Gibson of Macquarie University in Australia. “The safest option is for a breastfeeding mother to abstain from all alcohol until her baby is completely weaned off breast milk,” Gibson said by email.

Women who smoked cigarettes during the period when they were breastfeeding, however, didn’t appear to have children with different cognitive test scores than mothers who were nonsmokers. “That does not mean that it is safe to smoke,” Gibson said. “If women are have difficulty giving up alcohol and cigarettes they should talk to their doctor about ways to reduce their intake to minimize the impacts on the baby.”

Although prenatal alcohol and nicotine exposure have long been linked to cognitive problems in children, the current study offers fresh insight into the risks posed by exposure during lactation. Children exposed to alcohol in breast milk had lower cognitive test scores than other kids even when their mothers didn’t smoke or drink while pregnant.

The effects of alcohol were no longer apparent in test scores by the time children were 10 to 11 years old. One limitation of the study is that researchers lacked data on binge drinking during pregnancy, which is independently associated with cognitive problems in children.

It also wasn’t a controlled experiment designed to prove whether or how drinking or smoking might directly cause cognitive problems.

“Our understanding about the negative effects of drinking and smoking during pregnancy on health outcomes of offspring is pretty clear; however, there is limited information available concerning the effects of alcohol and tobacco in breast milk on the infant,” said Dr. Svetlana Popova, a researcher at the Centre for Addiction and Mental Health at the University of Toronto who wasn’t involved in the study.

“This study confirmed that maternal drinking while breastfeeding may cause dose-dependent reductions in children’s cognitive abilities,” Popova said by email.

It’s possible for both alcohol and tobacco can get into a woman’s blood stream and pass to her baby in breast milk, said Christina Chambers, a pediatrics researcher at the University of California San Diego who wasn’t involved in the study. “More needs to be learned about the pattern of consumption and the amount ingested by the infant in the many months when breast milk is usually the only source of the infant’s nutrition,” Chambers said by email.

Because breastfeeding has many health benefits for babies - including a lower risk of asthma, allergies and infections - new mothers who struggle to stop drinking while they are nursing should receive extra support to quit, said Dr. Lauren Jansson, author of an accompanying editorial and a pediatrics specialist at Johns Hopkins University School of Medicine in Baltimore.

“What we do not want is for this advice - not to consume alcohol during lactation - to be used as a punitive measure to attack women who choose to breastfeed, or to make fewer women breastfeed,” Jansson said by email. “For women that binge drink or are unable to curtail use during pregnancy or in the postnatal period, the availability of acceptable, non-punitive substance use disorder treatment that includes the infant is essential, as lactation can be supported in this group with appropriate intervention,” Jansson said.