The Middletons, it is well known, are a competitive family. So the announcement today that Kate Middleton, 35, is pregnant with her third child has increased speculation that her sister, Pippa Middleton, 33, may also be on the point of revealing a pregnancy.

Kate is thought to be as little as six weeks pregnant. She was forced to release the news early after being struck with severe morning sickness yet again, meaning she had to miss a planned engagement at the Hornsey Road Children's Center in London today.

Kate, 35, suffered Hyperemesis Gravidarum during her first two pregnancies and was hospitalized when expecting George in 2012.

The Queen was said to be “delighted” with the news, as were Kate's family, who may well soon have yet further cause to celebrate.

Pippa, who married the mega-rich city financier James Matthews in a lavish ceremony earlier this year, was widely rumored by friends and acquaintances to have made little secret of her desire to get pregnant sooner rather than later in married life.

And Pippa, who shot to fame after a scene-stealing appearance at the royal wedding in a tightly fitted McQueen bridesmaid's dress, has certainly been keeping a remarkably low profile since returning from an extraordinarily lavish honeymoon with her new husband, which saw the pair rack up thousands of miles of first class travel in a trip which circumnavigated the globe, taking in America, Taihiti and Australia.

Intriguingly, at the end of last week, Pippa closed down her company, PXM Ltd. She was due to file accounts but in a surprise move filed notice to have her company struck from the Companies House register instead.

Such a dramatic gesture suggests that Pippa is making big changes to her life - and may even mean she is effectively giving up work, at least for a spell, as PXM was not unsuccessful, recording substantial if not spectacular profits year on year.

PXM managed Pippa’s “publishing activities”, including her widely mocked (but lucrative) party planning book and journalistic endeavors.

Pippa has been notably absent the past few weeks from her usual haunts; Pippa and James are usually seen at London restaurants such as 5 Hertford Street just off Piccadilly or Mark’s club in Mayfair.

Pippa has also not been strolling the King’s Road (the couple have a $15m stucco-fronted townhouse just around the corner) as she often used to.

The last time she was photographed out and about was at the Winmbledon tennis championships in July.

It had been assumed that the couple were simply on holiday, however James is said to be back at work after his long honeymoon, and with the palace now revealing  that Kate is pregnant – and once again suffering the extreme form of morning sickness that blighted her previous two pregnancies – it may transpire that the reason Pippa has been out of view these past few weeks is because she has been mopping her sister’s fevered brow, and possibly comparing notes.

Us Weekly reported last week that a source had told them: “It’s long been a dream for Kate and Pippa to be pregnant at the same time. To share that would be amazing.”

Another source was quoted as saying, “Starting a family is something they’ve spoken about since long before they got married. Pippa’s always said how great it would be if they were to be expecting together."

Pippa and Matthews got married in a lavish ceremony near Pippa’s parents' home in rural Berkshire on May 20 this year.