The overjoyed Prime Minster said it had been "nerve-wracking" and a "bit of a shock" as his wife Samantha went into labour unexpectedly on holiday in Cornwall – and ended up having a caesarean section.
For the couple the arrival of their "unbelievably beautiful baby girl", the new life has an even more special meaning.
Just over eighteen months ago they experienced the most devastating blow anyone can face when they lost their son Ivan at the age of six.
The little boy had been ill all his life, suffering from cerebral palsy and a severe form of epilepsy, and needed round the clock care.
Because of the Camerons' determination to give him the very best quality of life, their home in west London had been specially converted to cope with his condition.
The house included a lift and a large playroom where he could spend time with his siblings, Nancy (born 2004) and Arthur (born 2006).
On February 25, 2009 the event they dreaded came: Ivan died after having a major seizure.
Of course, no one is suggesting that their as yet unnamed daughter can in any way replace their precious son.
But it will help them to gain a new focus.
That their focus would be family first was a decision Sam had already made during her pregnancy.
Shortly after the election the devoted mother announced that she would scale back her work commitments, cutting back on her hours at luxury stationers Smythson to concentrate on her children.
And the 39-year-old insists that her husband stop work early twice a week for a family supper. Mr Cameron, 43, also tries to make bath-time twice a week.
He has said that his older kids are "thrilled" and "bouncing up and down with excitement" at the prospect of meeting their little sister.
After all that the family have been through, everyone whatever their political persuasion, will be feeling they deserve this new reason for hope and happiness.