I saw and discounted this earlier !!

Name: William Stevenson
Spouse : Muriel Humphries
Date of Registration: Oct-Nov-Dec 1939
Registration district: Staines
Registration county (inferred): Middlesex
Volume Number: 3a
Page Number: 103

With the outbreak of WWII - strange things happened :-)

If Husband No 1 realises that this is not his child , perhaps they simply parted....
Although she would need to be "free" to re-marry.....

Name: Barbara Humphries
Mother's Maiden Surname: Humphries
Date of Registration: Apr May Jun 1939
Registration district: Brentford
Registration county: Hertfordshire, Oxfordshire, Buckinghamshire, Middlesex
Volume Number: 3a
Page Number: 322