"The dark line is called a "linea nigra" -- dark line! It is a common sign of pregnancy and usually happens only when the abdomen is somewhat distended. It is thought to be hormonally initiated and there is nothing anyone can do to get rid of it. Fair skinned women do not have this as often as women with darker pigmentation. Staying out of the sun does help to reduce pigment changes with pregnancy.
This line does not necessarily go away after the baby is born but it does get a bit lighter."
or you could have:
"Linea nigra (dark line running up your tummy)
This is a dark, vertical line, up to a centimetre wide, that appears down the middle of your stomach, often crossing the navel. It tends to appear around the second trimester and is caused by pigmentation in the skin where your abdominal muscles stretch and slightly separate to accommodate your baby as it grows.
This line of pigmentation will fade within a few weeks of delivery, although you may need to give it a gentle rub to remove any dry skin. You will notice that other areas with pigmentation ? such as your nipples, moles and freckles ? may darken too, but this will also fade with time."
or...
"Skin changes may occur in the second and third trimesters. Linea nigra is a dark, vertical line that may develop down the center of your abdomen. It is caused by the same pregnancy hormones that make your nipples darken. It darkens even more if exposed to sunlight, but it lightens after delivery"
or you could go with:
"Melanocyte-stimulating hormone (MSH) is another anterior pituitary hormone that increases during pregnancy. For many years, it's increase was thought to be responsible for many of the skin changes of pregnancy, particularly skin darkening (darkening of the areola, chloasma, and linea nigra). However, current belief attributes the skin changes to estrogen (and possibly progesterone) as well as the increase in MSH."
Or how about combining all of them?