The Countess of Wessex was taken by air ambulance to hospital where she underwent emergency surgery for an ectopic pregnancy.
She was said to be "comfortable" at the King Edward VII Hospital for Officers in central London after becoming ill overnight at her home, Bagshot Park, in Surrey.
She is expected to remain in hospital for the next few days. The Earl of Wessex, whom she married in June 1999, was with her after the operation, Buckingham Palace said.
The surgery was the first indication that Sophie Wessex, 36, was pregnant. Symptoms of an ectopic pregnancy, where the fertilised egg develops inside a fallopian tube instead of the womb, are most likely to show between the sixth and seventh weeks of pregnancy.
A doctor was called to the royal estate after she complained of abdominal pains. Within minutes of his examination, he called for an air ambulance. The Thames Valley helicopter left at 8am and landed in the grounds of Bagshot Park. The Countess was met by an ambulance near to the London hospital after a 20-minute flight.
The private hospital, founded in 1899, is favoured by the Royal Family. Princess Margaret was admitted there in October for tests.
A Buckingham Palace spokeswoman said last night: "Her Royal Highness the Countess of Wessex is currently recovering in the King Edward VII Hospital following an emergency operation. The Countess was taken to hospital by air ambulance after feeling unwell overnight. She is expected to stay in hospital for the next few days.
"The Earl of Wessex has been with the Countess following her operation. I cannot comment on the nature of the operation. Her condition is described as comfortable."
The most common reason for an ectopic pregnancy is damage to the fallopian tube, but in most cases the cause is not known. An ectopic pregnancy can cause persistent abdominal pain, sickness and diarrhoea and sometimes lead to collapse, according to the Ectopic Pregnancy Trust.
The trust said the chance of a repeat ectopic was about 10 per cent but it depended on the type of surgery performed. It said the loss of a fallopian tube did reduce future pregnancy success rates but about 65 per cent of women were able to conceive again within 18 months of an ectopic pregnancy.Reuse content