Per Google, it sounds like most women who get one will see it during the second trimester, around 20 weeks. It can appear earlier or later, and some women, especially lighter-skinned women, won't get it at all. :-)
Offhand, I didn't find a case of it showing earlier than 16 weeks but everybody's different. So, possibly?
BTW "What to Expect" says some docs think skin discolorations like linea nigra mean you're not getting enough folic acid. So if you aren't taking a prenatal vitamin, be sure to get some! http://www.whattoexpect.com/pregnancy/symptoms-and-solutions/linea-nigra.aspx
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