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It depends, every woman is different. Most women claim to see it later in their third trimester, which makes sense, because your body is preparing for the baby... other women, like myself, didn't start producing anything until my body was responding to me breastfeeding my newborn.
Likes Stefanies said it varies from pregnancy to pregnancy and from woman to woman. You could see it with your first pregnancy early in first trimester and then with your second pregnancy it doesn't happen until you are in your third. So if you aren't having it yet I wouldn't worry about it.