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Is Fennel Safe during Pregnancy?
Like most questions, this one seems like it would have a straightforward answer - but it doesn't, really.
As an herbalist and nutritionist with more than a decade of experience working with people who are pregnant or hoping to become so, I tend to take a fairly conservative approach to using herbs during this phase of life. I'd rather err on the side of caution during pregnancy.
Are fennel seeds safe for people who are pregnant? Yes and no.
Let's talk about the "Yes"
Most herbalists consider fennel (Foeniculum vulgare) an exceptionally safe herb during ordinary circumstances. It has a long history of culinary and therapeutic use - and it's quite tasty. Not that taste is a safety issue, but it does make fennel particularly appealing during pregnancy :).
It's on the FDA's Generally Recognized as Safe (GRAS) list. This suggests that it is safe to use as a food additive. (See this post for more on GRAS herbs and why this designation is frequently misunderstood.) Small doses such as those you get when using fennel as a spice when cooking or in very low doses as part of an herbal blend are almost certainly not problematic during pregnancy.
Remember, though, that GRAS is status doesn't specifically relate to supplements. It's very specific to food additives and lower-dose use. Fennel having GRAS status does not mean that it's safe to take indiscriminately during pregnancy.
What gives me pause?
Many of the traditional uses of fennel and those that have been explored in clinical trials suggest that it has some estrogenic and/or other hormonal activity. For example, studies suggest that fennel essential oil may improve symptoms of perimenopause, dysmenorrhea and oligomenorrhea/amenorrhea (see references list below) and it has a long history of use around supporting lactation, which is hormonally-driven.
Most of the research that's been done involves the essential oil of fennel and not the whole seed (technically it's a fruit, but anyway...). I do NOT recommend the internal use of essential oils in general during pregnancy. Definitely don't do that.
What we're talking about here is the use of the seeds, either whole, powdered, as infusions or extracts during pregnancy.
I found two studies involving fennel infusion that are worth considering in this context.
One randomized study used a higher-dose (11+ g) infusion of fennel plus a species of mint (M. longifolia) and vitex for the last 10 days of an induced menstrual cycle in 40 women with oligomenorrhea/amenorrhea. When compared to 40 other women who received medroxyprogesterone acetate instead, the fennel infusion acted similarly in terms of inducing menses, and in fact in the following cycle after discontinuation of both treatments, people who had been in the herbal group experienced on-time menses more often than those in the Rx group (50% v. 23.5%) and had a decrease in dysmenorrhea, acne and hirsutism, as well as significantly lower LH, total testosterone and free testosterone levels compared to the other group. This suggests that one of the herbs in the infusion combo or the synergistic effect of all of them did change steroid hormone production over an extended period of time (Falahat et al., 2018). (Note that I only had access to the abstract - boo, hiss! - so I couldn't tell the exact dose of fennel used here, but the authors do mention that it is the primary herb in this formula.)
A second randomized study compared 5g of fennel infused in water daily (except during menses) with metformin in women diagnosed with oligomenorrhea related to PCOS. Participants in the fennel group also received dry cupping. After 6 cycles, the two groups had similar results in terms of oligomenorrhea and BMI (cycle length decreased significantly, although more slowly in the fennel group, and BMI dropped in both groups), while those who received fennel + cupping had a significantly greater decrease in dysmenorrhea (Mokaberinejad et al., 2019).
These studies suggest that fennel infusion may interact with reproductive hormones.
Is this a bad thing during pregnancy?
No, not necessarily. We know that reproductive hormones such as progesterone, various forms of estrogens, etc. are dramatically higher during pregnancy than at other times of the life cycle. A small amount of phytoestrogenic herbs and/or herbs that alter reproductive hormone metabolism or production wouldn't necessarily make a dent or they may be beneficial in terms of having positive effects on uterine receptivity, preventing overactivity in the uterus, etc.
However, they might be problematic.
There's still a lot we don't know about how these hormones affect fetal growth and development, the initiation of the birthing process, and so forth. Some researchers and clinicians have suggested that fennel or constituents of fennel can reduce the contractility of uterine tissue. We aren't sure if this is happening, but it could be one mechanism by which fennel improves symptoms of dysmenorrhea. If this is true, do we want to play around with uterine contractility in the months leading up to birth?
One observational study of Italian women (Trabace et al., 2015) found that women who reported taking fennel throughout their pregnancies had a shorter gestational duration than those who did not (38.8 +/- 2.2weeks v. 39.1 +/- 1.6 weeks; p<0.05). If you know much about pregnancy, you'll remember that there's actually a lot going on in those last few weeks, and a shorter gestational period even by a few days may be more harmful than we understand. The same study also found a trend toward shorter length in newborns born to these moms, which wasn't significant.
Is this definitive? Nope, absolutely not. It may be that the moms were taking fennel because they were experiencing other symptoms that caused these differences. Or it may be that the fennel they were taking was not actually fennel, but was contaminated with something else, etc. In fact, a similar study (Faccinetti et al., 2012) of Italian pregnant women found that fennel was commonly used during pregnancy for 3 months or more and was NOT associated with an increased risk of preterm birth or other adverse effects.
However, in combination with what we know about the potential estrogenic effects of fennel, the studies suggestion potential effects on reproductive hormones, and the data above, there is reason to be cautious.
So, what's the conclusion?
Here's what I take away from this as a practitioner: small doses of fennel seeds - such as those found in foods as spices or used to flavor an herbal infusion - are likely safe during pregnancy.
With higher doses of fennel seed - in the range of 3+ grams daily, especially over the long term - I would think very carefully about the potential alternatives. In some cases those alternatives will be riskier than the unknown of taking fennel. In some cases, there are other herbs, supplements or strategies that you could try first.
I wouldn't recommend taking fennel during pregnancy "just because" or with the idea that it's a tonic of some sort.
Albert-Puleo, M. (1980). Fennel and anise as estrogenic agents. Journal of Ethnopharmacology, 2(4), 337–344. https://doi.org/10.1016/s0378-8741(80)81015-4
Bokaie, M., Farajkhoda, T., Enjezab, B., Khoshbin, A., & Mojgan, K. Z. (2013). Oral fennel (Foeniculum vulgare) drop effect on primary dysmenorrhea: Effectiveness of herbal drug. Iranian Journal of Nursing and Midwifery Research, 18(2), 128–132.
Cuzzolin, L., Francini-Pesenti, F., Verlato, G., Joppi, M., Baldelli, P., & Benoni, G. (2010). Use of herbal products among 392 Italian pregnant women: Focus on pregnancy outcome. Pharmacoepidemiology and Drug Safety, 19(11), 1151–1158. https://doi.org/10.1002/pds.2040
Facchinetti, F., Pedrielli, G., Benoni, G., Joppi, M., Verlato, G., Dante, G., Balduzzi, S., & Cuzzolin, L. (2012). Herbal supplements in pregnancy: Unexpected results from a multicentre study. Human Reproduction (Oxford, England), 27(11), 3161–3167. https://doi.org/10.1093/humrep/des303
Falahat, F., Ayatiafin, S., Jarahi, L., Mokaberinejad, R., Rakhshandeh, H., Feyzabadi, Z., & Tavakkoli, M. (2018). Efficacy of a Herbal Formulation Based on Foeniculum Vulgare in Oligo/Amenorrhea: A Randomized Clinical Trial. Current Drug Discovery Technologies. https://doi.org/10.2174/1570163815666181029120512
Ghodsi, Z., & Asltoghiri, M. (2014). The effect of fennel on pain quality, symptoms, and menstrual duration in primary dysmenorrhea. Journal of Pediatric and Adolescent Gynecology, 27(5), 283–286. https://doi.org/10.1016/j.jpag.2013.12.003
Mahboubi, M. (2019). Foeniculum vulgare as Valuable Plant in Management of Women’s Health. Journal of Menopausal Medicine, 25(1), 1–14. https://doi.org/10.6118/jmm.2019.25.1.1
Mahdavian, M., Mirzaii Najmabadi, K., Hosseinzadeh, H., Mirzaeian, S., Badiee Aval, S., & Esmaeeli, H. (2019). Effect of the Mixed Herbal Medicines Extract (Fennel, Chamomile, and Saffron) on Menopause Syndrome: A Randomized Controlled Clinical Trial. Journal of Caring Sciences, 8(3), 181–189. https://doi.org/10.15171/jcs.2019.026
Mohebbi-Kian, E., Mohammad-Alizadeh-Charandabi, S., & Bekhradi, R. (2014). Efficacy of fennel and combined oral contraceptive on depot medroxyprogesterone acetate-induced amenorrhea: A randomized placebo-controlled trial. Contraception, 90(4), 440–446. https://doi.org/10.1016/j.contraception.2014.05.001
Mokaberinejad, R., Rampisheh, Z., Aliasl, J., & Akhtari, E. (2019). The comparison of fennel infusion plus dry cupping versus metformin in management of oligomenorrhoea in patients with polycystic ovary syndrome: A randomised clinical trial. Journal of Obstetrics and Gynaecology: The Journal of the Institute of Obstetrics and Gynaecology, 39(5), 652–658. https://doi.org/10.1080/01443615.2018.1541232
Pattanittum, P., Kunyanone, N., Brown, J., Sangkomkamhang, U. S., Barnes, J., Seyfoddin, V., & Marjoribanks, J. (2016). Dietary supplements for dysmenorrhoea. The Cochrane Database of Systematic Reviews, 3, CD002124. https://doi.org/10.1002/14651858.CD002124.pub2
Samavati, R., Ducza, E., Hajagos-Tóth, J., & Gaspar, R. (2017). Herbal laxatives and antiemetics in pregnancy. Reproductive Toxicology (Elmsford, N.Y.), 72, 153–158. https://doi.org/10.1016/j.reprotox.2017.06.041
Trabace, L., Tucci, P., Ciuffreda, L., Matteo, M., Fortunato, F., Campolongo, P., Trezza, V., & Cuomo, V. (2015). “Natural” relief of pregnancy-related symptoms and neonatal outcomes: Above all do no harm. Journal of Ethnopharmacology, 174, 396–402. https://doi.org/10.1016/j.jep.2015.08.046