The Herpetological Conservation Trust has this excellent page on the etymology and folklore of adders, including:

“Old natural history books often tell how female Adders swallow their young to protect them from danger. This myth is even perpetuated by some countrymen who have spent their lives amongst Adders. This story suggests a degree of parental care which is sadly lacking in Adders. If she did attempt to swallow her own young the strong stomach acids would digest them. In all probability, this story originated when a gravid female Adder was killed with well developed young inside her.”  Read

Also I’ve added some statistics on adder bites here. Read