"Unlike someone who has grown up in the Royal Family and has been used to having close protection from an early age, it can be constraining" for Meghan, a source said.
The female police officer has never been named for security reasons, but she was the bodyguard who was photographed leaping into action during a security scare in Fiji during the couple's royal tour a year ago.
The bodyguard, who has not been named for security reasons, has been head of Meghan and Harry's security for less than a year.
A report from Daily Mail says the top female officer - the first woman to ever work as a key security officer for the Royals - is leaving Scotland Yard.
Meghan revealed that she is six months pregnant while she chatted to well-wishers, who lined the streets to get a glimpse of the royal couple during their first joint royal engagement of the year.
There has been no guidance either on where Meghan plans to have her baby.
Transfer: Willian takes final decision on joining Barcelona from Chelsea
We did well as a team and were good in many parts of the game but in the final third they made the difference. In my opinion, Willian has to stay with us because for us he is really a very important player, like Pedro ".
"But in her current role, she can't go anywhere without her protection team, and that's a massive constraining force on an individual like her".
Later, the couple will visit Tomorrow's Women Wirral before moving on to the Hive Wirral Youth Zone.
The blonde bodyguard was put in the spotlight earlier during Markle and Prince Harry's trip to Australia, Fiji and Tonga, when she was forced to intervene due to crowd management concerns.
Metropolitan Police Commissioner Cressida Dick is keen to raise the number of women in senior jobs.
"Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May  and are delighted to be able to share this happy news with the public", Kensington Palace wrote at the time of Markle's pregnancy announcement in October.