The expert answers: Tea is one of the most popular beverages in the world. The consumption of green tea is more popular in Japanese and Chinese cultures than it is in the United States. But due to the suggestions in the scientific literature that green tea may have beneficial properties for the prevention of cancer and heart disease, green tea is gaining popularity in America.
The difference between green and black teas is in the processing. Green tea is made by steaming and drying the tea leaves which allows for compounds known as "polyphenols" to be preserved. The polyphenols, most especially the catechins, are powerful antioxidants, and may be the compounds that give green tea its purported health benefits. In contrast, black tea is fermented during processing, reducing the ability of polyphenols to act as antioxidants. Green tea is available with caffeine or may be decaffeinated.
To play it safe, while there appear to be no ill-effects of green tea consumption, and, in fact, green tea may prevent certain diseases, it is not recommended that green tea be consumed in large amounts during pregnancy. The minerals, iron and calcium, are needed in higher amounts during pregnancy, and absorption of these two minerals may be affected by oxidation status and/or compounds in the tea such as phytates.
If you do like to enjoy an occasional cup of green tea, it is recommended that you switch to decaffeinated, since caffeine may act as a diuretic and deplete fluids.