Usually nipples point outward. Sometimes, they invert. When they persistently point inwards, they are called inverted nipples. This can be unilateral or bilateral. Many women are born with nipples that naturally invert (pull in) at times and evert (poke out) at other times. Other women find that this happens after breast feeding. Nipple inversion of this type is not cause for concern.
If your nipples have always been everted, however, and begin to invert for no obvious reason, this should be evaluated by your healthcare provider. Most causes of nipple inversion are not a cause for concern, but occasionally this is the first sign of a breast cancer. New nipple inversion is usually evaluated with a breast examination and mammogram as a first step.
With stimulation and nipple erection, most inverted nipples will evert. Occasionally, they remain inverted despite all efforts to evert them.
Other than for cosmetics, nipple inversion is not usually a problem. For breast-feeding, most inverted nipples will evert. Even those that do not evert may still function normally enough to allow for satisfactory infant nursing.