Pigment changes in pregnancy in the skin do occur. However there is no real evidence that there are significant change in most nevi. Those on the breast or abdomen will stretch as the skin stretches. Moles do not darken just in pregnancy, and changing color should be assessed like a mole changing color at any other time.
Nails can develop pigment changes, especially long lines, color in the cuticlel is not normal. Hyperpigmentation will regress after pregnancy, but not usually back to baseline levels.
The mask of pregnancy or chloasma or melasma is usually in the center of the face, but rarely can appear in arms or other areas of light exposure. Sunscreen can prevent melasma from appearing. You may want to wait a full year for treatment as some pigment still may resolve a year out.
Some women just develop demarcation lines, which are what they sound like, just a line where color of the skin suddenly changes. Demarcation lines are most likely to occur on the arms, no one really knows why, but these usually dissolve spontaneously.
Not all the pregnancy color changes are dark brown, some are red. Some women don't get as much pigment, but actually more like a rash. This one has to be the rash that is named essentially "a rash" for lack of a clearer description. So pruritic: "itchy" and urticarial : "allergic" reaction and papules: "bumps" and plaques: "flat red bumps", called PRURITIC URTICARIAL PAPULES AND PLAQUES OF PREGNANCY. The Brits like PEP instead of PUPPP. Their PEP stands for poylmorphic: "lots of different types" eruption of pregnancy. This condition is common: every couple of hundred prego women with one babe will get it. It's usually a condition of the first baby, and a bit more common in twins (about one in 50) . Extra stretch on the stretch marks or extra stretch marks might be a partially inciting factor. Faces are spared, but we don't know why. Steroid creams work well to control the itch. And after months of itching the moms often find it's completely gone a few days after birth! Sweet relief just in time for diaper duty!