Questionnaires were mailed to 8505 women in Canada who were randomly
selected from the Quebec Pregnancy Registry. In the end, 3183 were included in the study. Others were returned to sender, or excluded due to missing data or multiple birth pregnancies. This analysis included cases of women who delivered a baby < 2500 g; these were compared to controls in which babies weight > 2500 g. A total of 424 cases (13.32%) were analyzed.
The questionnaire sought to quantify any association between the use of herbs, but specifically flax, chamomile, peppermint or green tea, either alone or in combination, with the risk of low birth weight. The researchers focused on the use of herbs during the second and third trimesters of pregnancy. While there was a checklist of 40 herbs, they were also asked to list others they used but were not listed.
After adjusted for variables, there was no statistically significant increase in low birth weight with exposure to any herb during the last second or third trimester and specifically no association with the four herbs they were especially curious about. What they did notice was that smoking tobacco during pregnancy and giving birth to a female each increased the risk of low birth weight. It is also known that lower economic status increases the risk of low birth weight.
Commentary: This study was motivated by the observation that the frequency of low birth weight in Canada had increased from 5.7% during 2001-2002 to 6.1% during 2005-2006. The use of herbal products during pregnancy is reported to be 9%. In addition, there is some laboratory research that found that high doses of yarrow, fenugreek or flax may lower fetal birth weight if taken during pregnancy.
While this current study found no significant association between any of the most frequently used herbs and low birth weight there was some small association for both green tea and flax although not statistically significant. In a 2010 study of 392 pregnant Italian women, a higher frequency of threatened miscarriage (21.6%) and preterm labor (21.6%) was found in 37 women who regularly used chamomile.
In the spirit of caution, I would recommend avoid green tea, flax and chamomile during pregnancy. Herbs considered historically safe include ginger root, red raspberry leaves, nettles leaf, partridge berry, lemon balm leaf, oat straw and dandelion root. Ginger has the benefit of documented safety in published research studies.
Reference
Moussally K, Bérard A. Exposure to specific herbal products during pregnancy and the risk of low birth weight. Altern Ther Health Med. March/April 2012;18(2):36-43.
