My wife hit Silver in December, has flown 4 sectors since then all redemptions, 2 in F, one domestic connection, all with me (Gold), and one domestic reward single, which gave her lounge access.
After the F sectors in Feb, she was too pregnant to fly, and we're unlikely to be flying again before being she's downgraded on 8th December.
To retain Silver membership you need:
600 Tier Points
and
4 eligible flights
She did, however, received 271*2 avios from her Bronze bonus, having been promoted on November 16th.
BA should do a "maternity leave" thing for status, allowing you to take a few months "off".