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Babies of mothers who took antidepressants such as Prozac, Zoloft and Celexa during pregnancy show signs of early language development, according to a study from the University of British Columbia.

In contrast, babies of depressed mothers who were not treated with the most widely prescribed class of antidepressant drugs (selective serotonin reuptake inhibitors or SSRIs) show a delayed ability to attune to their native language, researchers found.

In the study, published on Monday in the Proceedings of the National Academy of Sciences, babies were divided into three groups: those exposed to SSRIs in utero, those of depressed mothers who weren't on SSRIs and those of mothers with no symptoms of depression.

Researchers tested how the babies responded to language at 36 weeks gestation, by measuring their heart rates, and at six and 10 months after birth, by measuring changes in heart rate and eye movement in response to sounds and videos of people talking in the babies' native and non-native tongues.

Janet Werker, a psychology professor at UBC and senior author of the study, explains that babies begin to respond to the sounds of language and the facial movements of talking very early in life. At about nine months of age, however, babies typically begin to ignore the consonants used in languages other than their native tongue as they focus on learning their native language.

In this aspect of language development, babies exposed to SSRIs were "at least three months advanced," Werker says.

But she notes that this accelerated learning "isn't necessarily a good thing." The study looked at only a few measures of the larger system of language acquisition. If some aspects progress before others, there is a risk of mistiming in the language system, which could affect overall language development, she explains.

Nevertheless, Werker cautions mothers against worrying about their decision to either accept or decline antidepressant treatment during pregnancy. "We don't know at this point whether these differences in timing [in babies exposed to antidepressants] have long-term consequences," she says.

The risks of untreated maternal depression are well documented, however. They include preterm birth, low birth weight, sleeping and eating problems and insecure attachment in infancy, and long-term cognitive, behavioural and emotional problems, notes Tim Oberlander, co-author of the study and a professor of developmental pediatrics at UBC and the Children and Family Research Institute at BC Children's Hospital.

"Non-treatment of maternal depression is not an option," he says.

Depression occurs in 10 to 20 per cent of pregnancies. Studies have found that major depression in pregnancy is the strongest predictor of postpartum depression disorder.

Nevertheless, many women go off antidepressants during pregnancy out of fear of harming their baby. A 2010 study found that half of pregnant women in B.C. who were on antidepressants stopped taking the medication within three months of conception.

The risks and benefits of SSRI exposure in babies remain unclear, however.

Studies have shown that babies exposed to SSRIs have blunted pain reactivity in the newborn period and increased risk of anxiety at age 3, Oberlander says.

But he adds that not all infants are affected by SSRI exposure in the same way, and some may benefit from a mother's treatment with an antidepressant during pregnancy.

Similarly, some mothers may find that alternative treatments such as cognitive behavioural therapy and regular exercise relieve their depression, while others may need antidepressants to improve their mental state, Oberlander says.

Until more is known about how babies are affected by antidepressant treatment, he says, mothers and clinicians must balance the potential consequences of untreated or poorly treated mental illness against the risks of drug therapy.