The social media star, whose real name is Ella Mills, told her 1.6 million Instagram followers that their daughter Skye Tessa Camilla Davan Mills was born on Saturday.
Posting a picture of herself cradling the newborn next to her husband, she wrote: “Born in the pool at home yesterday afternoon.
“The most powerful, surreal, totally extraordinary moment. Beyond grateful to my husband for being a hero, the best support I could have ever imagined, our midwife and doula for making us feel so safe and supported and @kghypnobirthing for the inspiration, wisdom and guidance she shared, which empowered us more than I can explain.”
She added that they are “completely in love”.
The couple have become parents 14 months after Dame Tessa, Matthew’s mother, died at the age of 70.
The former cabinet minister had been diagnosed with a glioblastoma multiforme brain tumour in 2017. She died in May last year after suffering a brain haemorrhage.
Ella and Matthew, who married in 2016, have been congratulated for their new addition by The Brain Tumour Charity.
The charity’s chief executive Sarah Lindsell said: “We are so very happy and thrilled for Ella and Matt on the birth of Skye, and send our warmest congratulations.
“We are honoured to have worked alongside them since Matt’s mother, Tessa Jowell, lost her life to brain cancer last year, and we are hugely grateful for their ongoing support as we honour her legacy by striving to improve the life of everyone diagnosed with a brain tumour.”
Food writer Ella, 28, rose to fame after founding her food blog Deliciously Ella in 2012.
She has released a number of books and has become a prominent force in the health food world on social media.
She is the daughter of former Northern Ireland secretary Shaun Woodward.