Firstly, warmest congratulations (I hope thats the appropriate phrase!).
She is obliged to inform the CAA Medical Branch immediately, they'll then either make arrangements for her to make an appointment to see them or her AME.
I'm not sure what the score is with aircrew, but with ATCOs the medical isn't automatically suspended straight away, but the individual usually has a restriction put upon the certificate requiring them to be supervised by another licensed, validated atco, (just in case the pregnant atco throws a wobbly).
With aircrew the restrictions may be more draconion due to the obvious implications should nature decide to throw an iron in the fire whilst the individual is flying.
In either case, the nearer to the date of birth, there will come a time when the medical certificate is suspended until after the birth and the mother passes another medical examination.
Good luck.
