Changes on breasts during pregnancy:
The hormones are the main reason behind many of the changes you experience in pregnancy. They change from day one and all through. Regarding the breasts, hormonal changes cause an increased blood flow to your breasts, and this can make them feel swollen and unusually sensitive to touch. For some it is similar to the feeling they get before the monthly period, only much more.
One of the earliest signs of pregnancy is breast tenderness, usually around 4-6 weeks, and lasts during the first trimester.
Other changes include increase in the size of the breasts around 6-8 weeks, and they continue to grow throughout pregnancy. What is common is to grow a cup size or two especially for first time pregnancies.
The breasts may feel itchy because the skin is stretching and you may have stretch marks on them.
You may also be able to see veins under your skin and your nipples will get larger and darker in color. After a few months, the dark circle around your nipples, areolas, will also get larger and darker.
There are small bumps that naturally exist on your areolas, a type of oil-producing gland, but in pregnancy they become more pronounced.
Your breasts go through these changes to prepare for nursing your baby.
Around the third month , your breasts start producing colostrum which is the special milk your baby gets when she first starts nursing from you. Some women start to leak a small amount of this thick yellowish substance in the last few months of pregnancy. Some start to leak earlier and some never do. It is normal.
There are some things you can do to reduce the soreness. Here is some advice from Baby Centre:
"Your best bet is to find a few good, supportive bras. Take the time to get fitted by a knowledgeable salesperson in a large department store or maternity shop.
You may find that bras with underwires are less comfortable now. To prevent chafing, look for soft material with no seams near the nipple. Cotton bras will be more comfortable and breathable than synthetic.
For extra support during the day, try a maternity bra. (During your third trimester, you might want to invest in a nursing bra, since you'll need one anyway if you'll be breastfeeding.) For nighttime, try a pregnancy sleep bra — a soft, nonrestrictive cotton bra available at maternity stores.
It's especially important to wear a supportive bra that fits properly while you exercise, because your breasts are heavier. A bra designed for exercise will provide the additional support you need and minimize discomfort.
Consider buying your bras with a bit of room to grow, since you may go up a size or two (both in cup size and chest circumference) as your breasts grow and your belly expands. Choose the ones that fit when the clasp is on the tightest setting, so you'll have some room to let them out. This is definitely a good idea if you end up buying a nursing bra before you give birth."
Baby Centre: http://www.babycenter.com/0_breast-changes-during-pregnancy_262.bc#articlesection2