Dark line down the thumbnail

My wife gave birth to our child a couple of weeks ago by way of a cesarean section. Just yesterday she noticed that on her right thumb nail, there are a pair of parallel grayish/blackish lines (kind of looks like one line though) that go from the be of the nail right up to where the nail meets the skin. No pain or anything. It is also only on this one thumbnail. Most of what we looked up says it could be melanoma skin cancer. She is in her mid 30s and she is Asian. We are not sure how long this has been there (she doesn't think longer than a few days) or if it has anything at all to do with pregnancy. Anyone have any idea ? Or should this be something we seek immediate medical attention for ? Thanks in advance !

Any medications taken during pregnancy? How wide is the line? Have you noticed any changes? 

I remember reading something about it related to pregnancy but can't remember what it is... Google "subungual melanoma" and it's probably on some research paper thing. 

No medications other than an iron supplement and prenatal vitamins early on in the pregnancy. The line I guess is about a little over 1mm wide and lengthwise is stretches across the nail. No changes yet since we've only noticed it a couple days ago. Subungual melanoma seems to basically be the scary form of melanoma that affects the nail. Hoping it's not that !

Iron supplement may be causing it but I'm not sure... If it's about 1mm there's less possibility for it to be malignant. Better go to your GP and get a referral for a dermatologist. You may wait a few weeks to monitor changes, but make sure you count the waiting for dermatologist in your total waiting time as well. 

If it's skin cancer, the fancy name for it is subungual melanoma. There are differential diagnosis as well, and googling it may help. And maybe look up Hutching's sign as well, as it is probably the number 1 sign a dermatologist looks for on your first visit. 

Relax, even if it's skin cancer, from what you've told me, it's still in its early stages. Worst case scenario is amputation of the thumb, which sounds scary, but gives your wife a 95% cure rate without anything else(no chemo, no radiation). 

  If you have any fears, go to your GP and get, no DEMAND, a referral to a specialist

So the dermatologist was very quick and simply said "I don't think it's anything bad" and to come back if the 2 lines get darker. Didn't give any explanation as to what it could be. We're not sure if we feel better, but definitely more confused. Should we go to another dermatologist?

You can if you want to. I wouldn't because I basically need to wait a couple months to get another dermatologist. Maybe go back/get another one and ASK what it could be even though it sounds rude.