The 34-year-old who is married to hedge fund manager James Matthews, announced her “happy news” in a fitness column she writes for Waitrose’s Weekend magazine.
She also revealed she passed the first trimester without suffering morning sickness, unlike her sister the Duchess of Cambridge, who experienced hyperemesis gravidarum at the start of all three of her pregnancies.
Middleton said she found keeping the news she was pregnant under wraps “hard” and she would use her monthly column to keep readers up-to-date from now on.
“I was lucky to pass the 12 week scan without suffering from morning sickness. That meant I was able to carry [on] as normal and continue my sports,” she wrote. “Who knows what the next few weeks have in store.”
Speculation over Middleton’s pregnancy first emerged in April, days before the birth of the Duke and Duchess’ third child, Prince Louis.
Middleton, who is a marathon runner, said she had noticed her body change and weight increase, and had stopped running to give her body a rest.
Her sister Kate was admitted to hospital and treated for a number of days for hyperemesis gravidarum during her first pregnancy in 2012. The condition, which causes severe vomiting, can lead to dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis. She suffered from the condition again while pregnant with Charlotte in 2014 and had to cancel engagements while she was treated by doctors at Kensington Palace.
Despite suffering another bout while carrying Prince Louis in April, Kate was able to carry out a significant number of engagements, but missed her eldest son, Prince George’s first day at school.