Many herbs are contraindicated in pregnancy due to not enough safety data being established in this sensitive time for the mother and foetus. Although Milk thistle (in small doses) is not directly contraindicated there are many differing trains of thought and much controversy also. I personally believe that unless there is a specific need for this herb during the pregnancy I would steer clear of its use.
A multivitamin supplement is designed to give small amounts of essential nutrients to boost nutritional status and reduce the risk of any deficiencies. For this reason many multivitamin supplements contain only the recommended daily intakes of the vitamins and minerals rather than giving high doses of these nutrients.
Generally it is not recommended to take any herbs (especially in the first trimester) while pregnant and especially ones which stimulate liver detoxification. Some herbs are recommended to take in the third trimester to build the strength of the uterus muscles and promote lactation. After childbirth Milk thistle is a herb used by many naturopaths in breastfeeding as it is a galactagogue which helps to increase the production of milk.
Please note that ‘Herbs’ refers only to medicinal plants not nutrients. Generally herbs and Vitamins are classified as separate entities. There is more safety data on Vitamins and natal products which contain them for use in pregnancy (BVitamins, Folate etc). It is also important to note that fish oils are also heavily promoted during pregnancy.
There are herbs which are considered safe if they are used in pregnancy for example Raspberry leaf in the third trimester, Ginger for morning sickness etc, the government stance however for general sale vitamins is that there is not enough safety data.
While some herbs are not considered to be harmful during pregnancy, their application and use is best advised by speaking to your own private Naturopath who is privy to all of the information on your own personal health.