Giant congenital mole on the back of a newborn baby. Birthmarks or naevi such as this are caused by overactivity or abnormality of melanocytes (skin cells that produce the brown pigment melanin). Giant congenital moles measure more than 10 centimetres across, and may be seen in around 1 in 20,000 births. Babies with giant congenital moles also often have smaller satellite moles. Most naevi are harmless and do not require treatment, however they may slightly increase the risk of skin cancer (melanoma) in later life.