Estrogen is actually a broad term referring to several different forms. The 3 main estrogens are known as E1, E2 and E3. The proper name for E1 is Estrone. This is a very strong form of estrogen and usually the one that becomes dominant in later life because the body continues to produce it. Estradiol is known as E2 and is also a very strong estrogen. However, it can become deficient in later life and you can experience symptoms and could possibly need to supplement it. E3 is Estriol (known as the gentle estrogen) Estriol is a very protective form of estrogen and is the estrogen most dominant during pregnancy because of its ability to protect the fetus.