London, May 22 (IANS): Mothers-to-be please take note! Babies who are exposed to anti-depressants in the womb are likely to have lower birth weight and gestational length, finds a new study.

The findings showed that infants exposed to selective serotonin reuptake inhibitors (SSRIs) -- commonly prescribed anti-depressants -- during two or more trimesters weighed 205 grams less than infants whose mothers were not exposed to any anti-depressants.

Also, the birth of such babies would, on an average, take place 4.9 days earlier.

However, "the biological mechanisms by which long term SSRIs exposure may affect birth weight remains unknown," said Katerina Nezvalova-Henriksen from University of Oslo in Norway.

The reasons for women taking SSRIs included not only depression and anxiety but also other neuropsychiatric disorders.

"Severe depression or depression not responding to non-pharmacological therapy may negatively affect the course of pregnancy and the pre- and post-partum period. The risks and benefits of SSRI therapy should therefore be carefully evaluated in each individual case," Nezvalova-Henriksen explained.

For the study, published in the International Journal of Epidemiology, the team included 27,756 siblings in the study, 194 of which were prenatally exposed to SSRIs.

They measured the effect of SSRIs and maternal depression on birth weight and gestational length.

By applying the sibling design, the researchers were able to address the unmeasurable and unknown family-level differences that may have been a source of bias.

The researchers also divided 7450 mothers into two groups -- one who used SSRIs during pregnancy and the other that did not use any anti-depressants.