The PM's press secretary Allegra Stratton also suggested cabinet reshuffle would see more women promoted.
Decision could mean Boris Johnson faces more pressure to appoint women to his top team.
Partner Carrie Symonds has given birth to a "healthy baby boy" but PM will remain in Downing Street until "later in the year".
"It was clear that I would probably be making more difference than at any other time in my career by donning my uniform."
"The idea of being a stay-at-home mum had always filled me with terror.”
More Than Half Of Dads Struggle To Secure Flexibility At Work – And It's Affecting Their Mental Health
Improving paternity leave is "vital", they say.
Some 100 employees of the Doctors Laboratory will strike next week.
"It’s not an affordable option for most working families," says TUC
Both mums and dads will be eligible for 52 weeks 🍻
A quarter are concerned for their job if they ask for more time off.