Britain's Prince Harry, left, and Meghan, Duchess of Sussex walk along Kingfisher Bay Jetty during a visit to Fraser Island, Australia. Picture: AP

London - As mother-to-be Meghan Markle, Duchess of Sussex, is learning, the first trimester – and often beyond – can be unexpectedly exhausting.

Consultant obstetrician Maggie Blott said: "During the first half of pregnancy, particularly the first three to four months, many women experience extreme fatigue, which is partly because the body is working very hard.

"A baby grows faster in the first three months of pregnancy than at any other time so a woman requires a huge amount of energy to support that growth. You also expect some pregnancy hormones to be higher during that period, which can also contribute to extreme fatigue. 

"It’s nature’s way of making sure a woman slows down to allow the body to concentrate on growing a baby. Extreme tiredness can often cause recurrent symptoms of nausea and vomiting – and the only thing to do in that situation is to rest.

"Anyone who has worked and been pregnant knows how difficult it can be to get up when the alarm goes off in the morning. And the only way to deal with is to go to bed earlier in the evening."

As for jet lag, Dr Blott said pregnant women can take longer to adjust. "I tell women who are travelling, 'Be patient, it will take a little time'."

But should a woman embark on a gruelling work schedule while expecting? Dr Blott said: "Look at doctors who work until they give birth - they all have busy jobs so it’s not crazy at all."