As a parent of 2 young children I know how difficult it is to know what is healthy and what may be potentially harmful. There is a lot of hype and pseudo-science out there when it comes to kombucha and cultures in general. It’s really hard to know what is right. I prefer to look at it with common sense and long term experience with these cultures.
When it comes to pregnancy and young children, I always like to err on the side of caution. It is true that culture ferments have been around for centuries. In Zimbabwe, they even use fermented foods during the process of weaning their children. However, in this day and age, most western societies have limited exposure to ferments or even bacteria in general. So for many, adding new bacteria to the micro-biome, altering the immune response, going through a detox can be a shock to the system. Usually that's a great thing, but just not during a critical stage development such as during pregnancy or nursing very young children.
Here are our biggest concerns as to why we don't recommend kombucha while pregnant or nursing:
Detoxifier and Immune Stimulate
My biggest concern with kombucha is that it is quite potent. It’s an amazing detoxifier and immunity booster. Two things you want in general, just not when you are pregnant. It pulls toxins out, but then it also has to deal how to get rid of those toxins. Being pregnant is quite taxing on the body already, adding an extra burden of pulling toxins is not something desired. Also, in order to protect the fetus, the immunity is lowered or in flux and adding a strong immunity stimulate like kombucha is not ideal either. For most pregnancies, it would be fine. But it may add a layer of burden you don't want during pregnancy.
Temporary Mineral Depletion
Another concern is the acetic acid produced during fermentation. Typically its an amazingly beneficial acid (think apple cider vinegar and all its glory), but it is a weak acid that needs to draw on minerals such as Calcium, Potassium and Magnesium to be neutralized. This only causes a temporarily depletion, but during pregnancy your body is in such high demand for those minerals, it could potentially be disruptive.
Another issue is improper fermenting which can could lead to contamination. Even though the risk is low, under fermented kombucha may not have enough acetic acid to fully protect against other potential negative pathogens such as Staphylococcus, E.coli, Salmonella. This typically does not pose a risk for the average adult, but the risk is much higher for pregnancy and nursing. Store bought kombucha has a lower risk, but it's not nearly as healthy. Also out of all the cultures, it has the highest risk of mold by a long shot.
Store bought kombucha has about .5% or less alcohol which is considered a natural ferment level. If you make kombucha at home it can range from about .5% to up to 2%. The .5% is not that big of a deal. It's a natural fermented alcohol level and it doesn't really feel like you are drinking a beer of wine. Sometimes with kombucha (especially with stored homemade kombucha), you can get quite a buzz and its much closer to an alcoholic beverage. The debate raging is whether small moderate amounts are okay vs no alcohol at all. The microbes in the gut naturally produce a small measure of alcohol, so its already part of the system. A large study found that drinking in moderation does not cause fetal alcohol syndrome. Moderation is probably okay, but once again I like to err on the side of caution. Also it does pass through breast-milk, so waiting a little to nurse after alcohol is probably a good option.
The fermenting process does use up much of the caffeine, but about 25-40% will remain (the general consensus is about 1/3rd remains). Considering you only do about 2-3 teabags per quart (or .5 to .75 teabags per cup) when brewing kombucha, it comes out to be significantly less than your average cup of tea. At most no more than 25mg even with black tea brewing. That's much lower than the guidelines of 200 mg or less per day. However, like alcohol there are minor risks involved with caffeine both to the fetus and nursing baby as it can pass through breast-milk.
There's no large scale studies to determine the potential effects of consuming kombucha during pregnancy and nursing. We prefer to err on the side of caution because we know the power of kombucha in immune stimulation and detoxifying. Anecdotally, kombucha tends to report the greatest side-effects such as diarrhea, rashes, dizziness etc. Much of that may be the body going through a cleanse, which is good, just not ideal during pregnancy due to upsetting the natural balance.
In our family we love our ferments and we alternate between the different ferments such as kombucha and kefir. During both pregnancies and nursing, we chose to stop drinking kombucha due to the more potent effects and then limited consumption mostly to grass-fed milk kefir. It has a milder effect, less acids, less alcohol, less chance of contamination and no caffeine.
Having said all that, I am a strong advocate of the age-old pregnancy wisdom that if your body is used to something, then its okay to keep doing it through the pregnancy. Changing or starting something radically new is not recommended as your body has to adapt which is hard to do while pregnant. Kombucha is no exception. If you've been drinking it everyday for years, then your body knows how to handle the effects and should be fine. Although I would still advise caution and perhaps drink in smaller quantities. Ultimately, I would also advise against starting kombucha or drinking more than usual during pregnancy and even nursing.