Do your babies have a soft bulge at the back of the head? I heard the soft spot at the back is supposed to harden by 1-2 months but my baby is 6 months old and it looks like it is bulging still.

It's called the fontanel if I'm not wrong. They look fragile but they are actually very well-protected by a strong membrane. The bones in baby’s skull will build up enough minerals to completely fuse together and those gaps will close. The back fontanel will close first; generally when baby is between 2 to 4 months old. The front will continue to grow well into the second year. It differs from baby to baby though. But the membrane protecting it us pretty sturdy so I reckon there's nothing to worry about.