Mary Ann Sieghart: It was good timing for the parents, less so for the baby

Prime ministers and babies never used to mix – except on the campaign trail, perhaps, where a quick peck on the cheek or chuck under the chin was allowed. When the youthful Tony Blair brought children to No 10 in 1997, that was a novelty in itself, even though the youngest was nine. But the fashion for electing ever-younger party leaders has made the prospect of pushchairs by the Downing Street door a much likelier – as well as happier – prospect.

It is not a house that is designed for children, though, let alone babies, as the Blairs can testify. Downing Street staff were initially appalled when the meticulously groomed garden became a glorified playground. And the Blairs had to move into the flat above No 11 rather than No 10, as it was the only one big enough to hold a family. Even so, it was too small for their unexpected addition.

When Leo was due to arrive in 2000, they begged Gordon Brown to let them take over two extra rooms from the Chancellor's flat (which he barely, if ever, used). He was tricky about it, saying that he had to protect future Chancellors' interests, and refused to release them until the very last minute. As a result, the detectives and driver who had to stay overnight in case Cherie went into labour were forced to sleep on the floor downstairs.

Giving birth was a trial for her. Unlike Samantha Cameron, who managed to take everyone by surprise on holiday in Cornwall, Cherie was pursued by photographers. Every time she looked out of her labour room window at the Chelsea and Westminster Hospital and saw the snappers, her contractions stopped. As a result, the labour was much longer than it had been with her other children. And she refused an epidural because she wanted to be able to slip out of the hospital early by the back door to avoid being photographed. Her screams horrified the detectives next door, one of whom was pregnant herself.

Tony Blair took paternity leave, but he looked grey with tiredness for the first few months after Leo was born. Cherie insisted that he do his share of night feeds, even though he had a country to run during the day. He was by then in his late forties, a relatively old father. Still, when I remarked to a Downing Street aide that it must be very hard for a prime minister to manage with a baby, I got a tart response. "At least he lives above the shop and can go and see his children whenever he likes. I've barely seen mine for years."

Compared with the Blairs, the Camerons have timed their delivery well. Because the baby was early, David can now afford to take the next couple of weeks off and still have plenty of time to prepare for the Tory Party conference. His new daughter may not thank her parents as she gets older, though. Because she has a late-August birthday, she is doomed to spend her entire childhood and adolescence as one of the youngest pupils in her year.