This is very common in pregnancy. Skin darkening is caused by the effects of your hormones stimulating melanocytes production which causes pigmentation patches on the skin. On the face, it is called Chloasma and used to be called the 'mask of pregnancy.' Freckles, scars, and areas on your face can darken. On the abdomen it is called the linea nigra where a dark line develops from the umbilicus down to the pubis area. The arerola (dark area) of the breasts will also darken.
It does not harm you in any way and usually disappears several months after you have your baby.
Will I get stretch marks?
A common question that women ask. Striae gavidarum or stretch marks occur usually on the breasts, abdomen and thighs of the pregnant woman. Unfortunately, what is not known is what women will get them and what women will not. Stretch marks are caused by the elastic collagen tissue under the skin stretching and tearing and forming red stripes over the abdomen.
The good news is that after you have the baby, the marks will fade and eventually becomes faint silvery lines, although unfortunately the skin texture will always be there.
I have been trying to find some online research about the use of olive oil/cocoa butter etc., helping to reduce the risk of stretch marks but no evidence has proven conclusive. Vitamin E is a vitamin that is found in certain skin pregnancy products and is said to help with the repair scar tissue. Here is a link to a website that explains stretch marks, they also give a list of foods that contains vitamin E.