That dark line, the Linea Nigra, is caused by higher levels of melanin – we’ve covered melanin back in week 36 with; “The Mask of Pregnancy” Is Caused by Melanin!
Before you were pregnant the Linea Nigra was called the Linea Alba and it ran from your pubic bone up to your tummy button. The higher levels of melanin cause discolouration of the skin. Many pregnant women (as high as 70%) notice discolouration, for some it is just a “radiant glow”, while for others it can be more noticeable. Darker areas skin often become even darker, such as nipples, moles, freckles and even scars.
The changes in skin tone are known as hyperpigmentation and like most things during pregnancy, it’s all down to the hormones, they temporarily stimulate a production increase in melanin.