My husband is Indian, I am white. Our oldest daughter, who takes more after my dh in complexion, has the line from her belly button on down. She was also born with a mongolian birthmark but that faded with time. The line is still there but very faint. I suspect when she get pregnant later in life it will be dark. My mother in law said the line is normal in some people in the family.
one of my best friends is black/white biracial. She has several brothers and sisters and also has a lot of biracial pride. She says (in her anecdotal experience) it is very common for black/white biracial children/people to have birth marks and variations in skin tone...and sometimes darker skin areas can mean that they are going to get darker complected as they get older. especially if those marks are along their median line (for lack of a better word) ...spine/below the belly button/genitals and is not a defined birth mark but an area of darker skin
I have another Latina friend who has said in the past that with hispanic babies you can tell if they are going to get darker complected if they have areas of darker skin in those same places
It sounds totally normal to me but if you are worried about it, you can always talk to your ped
you know, I don't know. I can tell you that in my family, we are a good portion Cherokee and of all the cousins in my generations, those of us who burned the easiest as kids grew up to be the lightest complected. My mom and littlest brother never ever burn, are more olive skinned than I am, and have much darker skin tones than i do even in the winter. I on the other hand barely tan. I burn. I am very pale as an adult, though my little brother and I appeared to have the same skin tone when he was a baby/toddler. He didn't start getting darker till he was 4 or 5. But that is just our family's experience