Unknown to the public at large, many vitamins today are produced through total chemical synthesis – a process whereby a complex organic molecule is synthesized from simpler, petrochemical-derived precursors, usually without the aid of a biological process.
What this means is that many vitamins are essentially being produced from oil. Despite sharing certain chemical resemblances with the nutrients they were designed to be technical facsimiles of, these pseudo-vitamins are being consumed at the rate of millions of lbs annually – and often sold to the consumer as “all natural.” .
One of the more disturbing permutations of oil-derived “vitamins” is retinyl palmitate (also known as vitamin A palmitate ), a synthetic vitamin  form of vitamin A. It has been linked to birth defects in a wide range of animal studies. Some of this research you can view first hand on GreenMedInfo.com. For additional verification the Environmental Working Groups’ website Skin Deep lists a wide range of scientific references and has determined that it has a high risk for developmental and reproductive toxicity .
A human study published in the New England Journal of Medicine in 1995 showed that women who consumed 10,000 IU of preformed vitamin A  from supplements per day had 4.8 times the rate of birth defects than those who consumed 5,000 IU per day while pregnant. The increased frequency of defects was concentrated among babies born to women who consumed high levels of vitamin A before the seventh week of gestation.2
Despite this research, consumers, are mostly kept in the dark. Even more disturbing is the fact that many mass market and prescribed prenatal vitamins contain the preformed, birth-defect associated forms of vitamin A. 
Ultimately, pregnant women need to get their vitamin A through vegetables . A 100 gram (a little over 3 ounces) serving of kale , for instance, has a whopping 10302 IUs of vitamin A from beta-carotene,1 a form not associated with birth defects, and which can be consumed in plenty in food form without overloading the system.
Also, consumers and health practitioners alike should seriously reconsider the risks associated with the use of mass market prenatal vitamins that commonly contain synthetic vitamins such as dl-alpha tocopherol  (“vitamin E”), another petrochemical derivative, as well as highly toxic chemicals such as sodium selenite /selenate, and cupric sulfate , which is commonly used as a pesticide.
Read the full article here: http://www.greenmedinfo.com/blog/why-birth-defect-linked-vitamin-used-prenatals 
1 NutritionData.com, Kale, Raw 
2 Teratogenicity of high vitamin A intake. N Engl J Med. 1995 Nov 23 ;333(21):1369-73.