Estrogen and progesterone are the chief pregnancy hormones. A woman will produce more estrogen during one pregnancy than throughout her entire life when not pregnant. The increase in estrogen during pregnancy enables the uterus and placenta to: improve vascularization (the formation of blood vessels). Hormonal changes taking place in pregnancy will make your nipples and the area around them go darker. Your skin colour may also darken a little, either in patches or all over. Birthmarks, moles and freckles may also darken. Some women develop a dark line down the middle of their stomach, called 'linea nigra'.