I personally haven't been told this, however my aunt has when she was pregnant with my cousin Holly (now 10).
Basically when Holly was born she had exactly what they said a long thin skull, mening her head width ways (like from ear to ear) was really thin but it was really long from forehead to back of head. I dont know why, ive never asked and was never told as I was only 9 or so then. I know that she got n operation when she was abotu 6 months or so though to fix her skull and make it a more normal shape as when her head was growing it was growing longer rather than bigger...wasnt a life threatening op or anthing and from what I can remember she was out of hospital quickly.
This could be what they meant...or it could be something completley different such as that your baby will be big.
Anyways, whatever it is im sure its nothing to worry over, if it was they'd have given you more information on it i think
Good luck!!
xxxxx