It is true that moles can get bigger during pregnancy and this is because of hormonal changes in the pregnant mother. The clinical term for a mole is a melanocytic nevus (http://en.wikipedia.org/wiki/Melanocytic_nevus ), which, if you can’t work out the Latin, means a ‘sharply defined blemish of the skin made up of melanocyte cells’. Melanocytes are the cells in your skin that contain the skin pigment. During pregnancy, the levels of certain hormones increase and this can activate these melanocyte cells – causing them to grow.
But which hormones are causing the effect? The first suspects are the Melanocyte Stimulating Hormones which, as the name would suggest, affect the melanocytes that make up moles. The Melanocyte Stimulating Hormones are produced by the pituitary gland, a tiny hormone-making organ located in the bottom of the brain. During pregnancy this gland grows (it can increase by up to 50% in size), meaning that hormone production also increases (http://www.rnceus.com/hormone/antpit.html ). Therefore, as the moles scattered across the skin receiving more hormones, their growth will be stimulated and the moles grow larger.
Is this the end of the story? Not quite. During pregnancy, there is often skin darkening in other areas of the body too. This skin darkening cannot be totally accounted for by the increase in the Melanocyte Stimulating Hormones. Research in the 1990s showed that it is the female sex hormone oestrogen that also causes skin pigmentation (McLeod, Ranson and Mason 1994), hence the rise in this hormone (as well as Melanocyte Stimulating Hormones) may also stimulate moles and cause them to grow.
Hormones have a lot to answer for!