May botanicals are thought to be mild both in their effects and their side effects. However, there is very little data on the safety of herbal medications during pregnancy, according to a January 2009 review article published in "Alternative Therapies." As an herb, fennel is used in small amounts to add flavor to food, but if you're using it for medicinal purposes, consult with your doctor to discuss safety.
Fennel and Pregnancy
Traditionally, in Mediterranean areas, fennel seed has been used for a number of ailments and is believed to have diuretic, analgesic and anti-inflammatory effects in addition to preventing muscle spasms. Herbal mixtures containing fennel extracts may also be helpful for babies with colic, according to a July 2012 article published in "Evidence-Based Complementary and Alternative Medicine." Lactating women can safely add fennel seed to the diet, according to the authors of the review article in "Alternative Therapies," who suggest candied fennel after a meal. There are no indications for its specific use during pregnancy, however.
Concerns and Cautions
There is very little known about the safety and efficacy of fennel seeds, and the literature offers contradictory evidence. Fennel contains a carcinogen called estragole, which in high doses has been shown to be toxic to the liver of rats and mice. However, the seeds also contain a substance called anethole that has anticancer and antitumor activity. Women often do not share information about their use of herbal supplements during pregnancy, according to "Alternative Therapies," but it's important to talk to your doctor about what you're taking to monitor for adverse effects.