One became a mother again just yesterday; the other has a child on the way. Doubtless Sarah Brown watched television pictures of the scene outside St Mary's, Paddington - where Samantha Cameron had her Caesarean section - knowing that it will be her turn next to give birth amid the kind of publicity that was once reserved for new royal arrivals.
There is more that links these two women than overlapping pregnancies. With so little political substance now dividing the Labour and Conservative parties, the next general election looks set to be a personal duel between Gordon Brown and David Cameron.
While the country is asking which man would make the better Prime Minister, an uncomfortable level of scrutiny will be directed at their families. In these celebrity-obsessed days, it has become the custom to judge a politician by his private life. The Browns and the Camerons will struggle to protect the privacy of their young children. For the wives, there will be no escape.
It remains to be seen who will handle the pressure better, but we can be reasonably sure who will find the ordeal more vexing. Sarah Brown has already had eight years in the public eye. She hates unwanted intrusion, is wary of journalists, and reportedly finds it embarrassing when she is compared to the older, more experienced and less publicity-shy Cherie Blair. There is no reason to think that she will enjoy being compared with the younger, self-confident Samantha Cameron. Even so, yesterday, while her husband raised his profile as Prime-Minister-in-waiting with a series of key statements on subjects such as terrorism, Sarah Brown made one of her rare forays into the limelight the launch of a campaign against domestic violence.
Should the two women ever meet for a heart-to-heart, they will find there are glaring differences between them, some of which will have been contributed to their turning up on opposite sides of the political fence. Their family backgrounds, for a start. Sarah's was reasonably comfortable; her mother was a teacher, her father worked for a publisher, and she spent much of her early childhood in Tanzania. But these beginnings were nothing like as upmarket as Samantha's.
There is a famous story about Mrs Cameron: that, at a Conservative conference, she claimed to have grown up "outside Scunthorpe". That was true, but misleading. Her father, Sir Reginald Sheffield, owns Thealby Hall in nearby Thealby; Sutton Park, a stately home near York; and the nearby 1,900-acre Normanby estate.
As the young Samantha Sheffield emerged from Marlborough College, she went on holiday in Bali and acquired what has since become a notorious tattoo on her ankle, depicting a bluey-green dolphin with its back arched. She already knew David Cameron, whose sister Clare was her best friend at school. She was married at 25.
Sarah Macaulay, as she was then known, was past 30 when she met Gordon Brown, in 1994, through her PR firm Hobsbawm Macaulay, which she set up with Julia Hobsbawm, an old friend from their days at Camden School for Girls.
For all the contrasts in their respective families, the two women have much in common. They both experienced family break-up before they were eight years old (as Cherie Blair also did), which may explain their fierce determination to hold their families together against the pressures of public life.
Those pressures were heightened by heartbreaking setbacks - in Mrs Brown's case when her daughter Jennifer died a few days after her birth; in Mrs Cameron's when her first child, Ivan, was found to be severely disabled.
They also have in common a shrewd political judgement, an ambition to see their husbands do well, and the social skills of good hostesses. People who have met both women say that they are quick to put others at ease and clever at remembering important details, such as the names of guests' children.
It is a curious quirk of the British political system that the wife of a Prime Minister (or leader of a major political party) has less status than the Lady Mayoress of a provincial town. There is no "First Lady" in the British political system, no public role for the mother of the Prime Minister's children. A generation ago, it was even possible for a diffident wife, such as Audrey Callaghan, to stay out of the public eye altogether.
But modern curiosity about the private personalities of public figures, and the demands of a range of voracious news outlets, have made that almost impossible today. Like it or not, Sarah Brown and Samantha Cameron are not private individuals. They are the first ladies in waiting.
BORN 1971, the daughter of a knight and a viscountess.
EDUCATION AND CAREER Marlborough College, Bristol University, BA in Fine Art. Joined the Bond Street stationers Smythson, where she is now creative director.
LIFE BEFORE MARRIAGE At university she hung out with trip-hop star Tricky, learning to play pool with him. Dubbed a trustafarian for her rich clique's fondness for murky pubs and underground music. Spent gap year in Bali, acquiring a tattoo on her ankle.
MARRIAGE Her best friend, Clare Cameron, introduced the then 16-year-old Samantha to her 21-year-old brother David at the family's Italian villa. They dated while Samantha was at university and married on 1 June 1996 in Oxfordshire. They have three children - their first-born, Ivan, four this year, has epilepsy and cerebral palsy. The couple take it in turns to sleep in his room in case he has a seizure. They have homes in Kensington and Oxfordshire.
PERSONAL STYLE Samantha brought a touch of glamour to the Tory party conference last year by mixing high-street and designer style. The £875 crocodile-finish Smythson handbag she wore soon sold out. Like Sarah Brown, she's into alternative therapies and enjoyed an aromatherapy massage while pregnant, although it's not recommended. Unlike Cherie Blair, she has no political ambitions. "I am not going to tell Dave what to say," she says. "But I want to be there for him."
STRENGTHS Camera-friendly, media-savvy, knows how to handle life in the spotlight.
WEAKNESSES Quick to downplay her privileged background, which may undermine her appeal. Can the public trust someone so quick to deny her past?
BORN 1963, father in publishing, mother a schoolteacher.
EDUCATION AND CAREER Acland Burghley Comprehensive and Camden High School for Girls, then Bristol University where she read psychology. Joined Wolff Ollins PR firm before co-founding Hobsbawm Macaulay PR with school friend Julia Hobsbawn. Founded PiggyBankKids charity in 2002, and yesterday launched campaign against domestic violence.
LIFE BEFORE MARRIAGE At school she was prominent in a clique known as the "trendies". Serious about her career, she thrived in the world of London PR. Respected and well-liked, she moved in left-wing, arty circles and has retained an air of independence.
MARRIAGE Met Gordon through Hobsbawm Macaulay when Labour was among its clients in the mid-1990s. First met Gordon properly on a plane travelling to Scotland. They had been together for four years when they married in August 2000 in Fife. Sarah gave birth to Jennifer Jane in December 2001, but she died when she was only 10 days old. Their second child, John, was born on 17 October 2003. Last month they announced they were expecting a third child in July.
PERSONAL STYLE Publicity shy. The split with her business partner Julie Hobsbawm was an unhappy episode for both women. Last year, hoping to become pregnant at 42, she underwent acupuncture fertility treatment.
STRENGTHS Shrewd judgement, careful not to make mistakes. No seeker of publicity. Highly regarded within her circle of friends.
WEAKNESSES Her reluctance to lead life in the public eye will make life in the spotlight hard. She has always disliked being compared to Cherie.
Amy WinstonReuse content