Kate Middleton and Prince William are allegedly expecting their third royal baby as reports revealed that the Duchess of Cambridge is pregnant again.

After giving birth to Princess Charlotte four months ago, the 33-year-old Kate Middleton is now expecting her third baby, a source told Star magazine as reported by Radar Online.

"She's in the early stages of pregnancy with baby number 3," the insider said. "She's not taking any chances."

This came after recent reports claimed that Kate Middleton has been rejecting invitations because she needs to take some rest following her pregnancy, OK! Magazine said in their report.

"Kate has the Queen's blessing to forgo royal duties for at least two months, until she overcomes the treat of sickness," the source added.

This has allegedly resulted to Prince William's solo appearance during high society gatherings and occasions, including a recent wedding last August 15.

Kate Middleton's sister, Pippa, was there to be her representative and to escort Prince William.

"Pippa was delighted to escort William to the wedding. Kate knows she can always rely on her sister and doesn't hesitate to call for her assistance," the source said.

According to a report from the Hollywood Life, Kate Middleton and Prince William's third child is due either by April or early May next year.

Before the Duchess of Cambridge reportedly decides to show off her baby bump this coming New year, an insider revealed that Kate Middleton wants to enjoy a vacation in her bikinis first.

"She knows the paparazzi will try everything to get the first photos of her bulging swimsuit - that's why she wanted to take an earlier vacation. She figures her bump won't be prominent in Nov., so she can relax in her bikini without her pregnancy being so obvious," the source said.

Kate Middleton and Prince William are parents to Prince George, 2, and Princess Charlotte, heir five-month-old daughter.