Ones at the bottom of pic look like stretch marks..... hard to tell though. Your skin might be tighter there because of the surgery so stretching more than other areas. Maybe get mw to take a look if it's worrying you.
I got the same on pre-existing scars, it's totally harmless and normal. They stayed brown the whole pregnancy and faded back afterwards. Its a increase in melanin - you produce lots whilst pregnant so your nipples darken, to create contrast between them and your boob to help your baby see them. However, this is not selective and will sometimes affect scars - it also means you'll tan easier! However, I am not a doctor and if it worries you, check with your GP
They will fade to hardly noticeable. Just use lots of bio oil after babies born and they go quite quickly. My stomach looks like a road map at the mo but not too bothered as they will fade soon enough. Xx