i had linea nigra during my first pregnancy, but none for this one. my tummy was wide for #1 and is also wide for the current #2, and both are boys. in fact, most of our friends and colleagues lost their bet when they guessed that #1 was a girl (we didn't find out the gender until delivery day). and for both pregnancies, my skin and hair condition actually improved.
so my conclusion is it is old wives' tales!!! ;p