When my two sons were born they each had those pink-red splotches on the backs of their necks up almost into the hairline. I thought it was a family birthmark until I noticed lots of babies have those. Has anyone else noticed those? What are they and why do they appear on just the back of newborn necks? I did notice that they tend to fade over time.
They’re called “stork bites”. It’s a birthmark, and it’s pretty common for babies to have them (I got this info by entering “stork bites” into Dogpile.)
I assumed that they were minor bruising caused by the pressures of childbirth, but it looks like I’m wrong.
I thought they gradually moved up into the hair-covered part of the head. No, not like walking, but as the skin stretched this part of the neck would stretch up under the hair.
And now that I’ve said that it seems dumb, too.
Anyway, I used to ride public transportation in Washington DC and often noticed the stork bites in the hair above the necks of the guys around me.
Maybe it is an X-Flies thing.
Or maybe X_Flies, Y & Z.
Maybe its the forceps pull mark?
Macular stains — Also called “angel’s kisses”
or “stork bites,” these flat pink marks are the
most common vascular birthmarks. They occur
on the forehead, eyelids, tip of the nose, upper
lip, back of the neck and other locations. They
are harmless and almost always fade by age
two.
Cute stuff from the Mayo Clinic via a search engine - under stork bites.
I’m not sure I’d want to know what was lurking in the hair above the necks of the guys around me on public transportation. 
Nope, my wife had both of our kids at home and the younger one had 'em. I thought that they were brought on by the pressures of childbirth (he was an anterior birth and was a pretty big kid), but I haven’t found this mentioned anywhere as a cause.