Mrs Williams' first tweet informed her 16,000 followers that her waters had broken. "It wasn't like Charlotte in Sex and the City," she wrote yesterday. "Now timing contractions on an iPhone app."
 
 She continued to tweet throughout the labour, describing, around two hours later, how timing the contractions was fun until it "got painful". As her labour progressed and the pain worsened, she managed to hold her contractions at bay just long enough to tweet: "Epidural, yes please."
 
 There was a long gap between that and her next tweet, sent shortly before her baby was born: "The heartbeat monitor soothes the silence of a room that will shortly be anything but silent."
 
 Mrs Williams' husband, Evan, Twitter's co-founder and chief executive, was left to herald the arrival of their son. "Yes!," he tweeted. "@sara gave birth to a perfect baby boy. Both are well. 8 pounds, 21 inches! Smiles all round."
 
 Neither Evan Williams nor his wife has yet uploaded a photo of the new arrival to Twitter, though an exhausted Mrs Williams did inform her followers that her husband was "changing the first diaper".