Darkening of nipples ok if pregnant?

Normal. That's normal. Hormone levels change in pregnancy. One of these effects is increased melanin which darkens parts of the body.

Related Questions

What can cause erect nipples that aren't sore at all, prominent montgomery glands, and darkening of areolas besides pregnancy?

Many things. If your nipples are always like that then this may just be how you were built. Cold, tight bras, friction, hormonal changes can all cause a temporary change in nipple appearance. Schedule a visit and have a doctor help if you are concerned.