The snap, showing a big white bra laid out across a table, was accompanied by the tweet: "Fearne's dad bought her something very weird from a car boot sale yesterday - not funny dad!"
The DJ and television presenter, 30, announced at the beginning of August that she and boyfriend Jesse Wood were expecting their first child together.
In a statement on Cotton's website, the star had gushed: "Jesse and I are very happy to announce we are having a baby. We are over the moon, morning sickness aside (me, not Jess) and looking forward to the little Cotton Woods arrival."
Earlier today (August 20), Cotton showed off a hint of a baby bump in a white loose-fitting dress as she arrived for work at the BBC Radio 1 studios after a week off.