Mrs Johnson, 37, posted a picture of herself and her newborn daughter Poppy Eliza Josephine on Friday in a hospital bed.
“Being hospitalised for two nights for severe dehydration was not on my postpartum bingo card,” she captioned the Instagram post.
Mrs Johnson urged other “breastfeeding mums” to make sure they eat and drink enough “in this heat”, especially those who are “clusterfeeding”.
Poppy was born on 21 May, becoming the couple’s fourth child after their son Frank, born in July 2023, daughter Romy, born in December 2021, and son Wilfred, born in April 2020.
In a separate Instagram story, Mrs Johnson described an “honestly brutal week”.
“Mastitis (me), reflux (her), dehydration (me). What a pair we are!,” she said.
“But thank you for all the kind messages, especially all the brilliant advice on reflux. Really appreciate it and made me feel way less alone going thru (sic) it all. And as ever, thanks to our amazing NHS.”
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The NHS recommends drinking plenty of fluids while breastfeeding – and avoiding caffeine and alcohol to stop their effects being passed on to the baby.
Having a drink nearby when mothers stop to feed is advised, as is water, lower-fat milk, and low-sugar drinks.
The UK Health Security Agency (UKHSA) has issued an amber heat health alert for the next four days.
Although not a public weather warning, it advises health and social care organisations of possible dangers to their patients and facilities.
Temperatures could reach 34C on Monday – with a 20% chance of beating the hottest June day on record of 35.6C from 1976.
The likelihood of record-breaking temperatures could increase over the weekend as the day approaches.
