Possible adoption

[Cockneyrebel]

My cousin's mother (now deceased) Mary Alexandra nee Dell Street married Clifford Ernest Howard Street Q2 1936 in Worthing, Sussex. The couple had 2 children 1937 & 1938 and suddenly Clifford passed away Q4 1939 in Worthing aged 27-28. It is thought that Mary was pregnant at that time and that the baby was born after Clifford's death in 1940. It's obvious I guess that Mary having already 2 children couldn't keep a 3rd and family information believes that the 3rd was adopted. Mary married again in 1942 and she never mentioned anything about it to her other children or anything at all about Clifford, so it's a mystery. An aunt kept a birthday book and there is a date of 15/3/1940 but no name so this might be the adopted child?
Would the child's birth have been registered as its adopted surname or father/mother's name?
Another DNA relation has kept this secret but lived in Canada and was sworn to secrecy by Mary and wouldn't reveal it even after Mary's death.
Cr

[Gan Yam]

The child would be registered with its birth parent/s name.  My mother in law was adopted and the entry of her birth is in her birth parents name.

[Milliepede]

You can look up birth registrations covering that date using parents surnames (you probably already know that)
to look for a likely candidate. 

[Cockneyrebel]

Yes, I tried looking for a birth record for that year and place with the 2 surnames but to no avail. As it was during the War could the adoption been done and registered by the adoptive parents secretly?
Cr

[scotmum]

If she was pregnant, could she have given birth in a different area and under her maiden name? Free BMD has one birth in right year/quarter, of a child with same surname as mum's maiden name, and mother recorded with same surname.

[rosie99]

Interestingly that 1940 birth married in 1960 with 2 surnames for them showing on the index.  As they are probably still living we should not give information on here.  Births for children show other surname as MMN.

[Cockneyrebel]

Very interesting and can only be one marriage as the 1952 one would be too early. looks like the child had 2 surnames at marriage too according to Anc records.
Cr

[rosie99]

The 1952 marriage also has two surnames for the female 

[AntonyMMM]

As it was during the War could the adoption been done and registered by the adoptive parents secretly?

Not if it was done through the proper legal adoption process no.

If they were given a baby and claimed it was their own, then they could falsely regsiter it as such but it wouldn't be an adoption - just a normal birth record (albeit one with false information).