The singer-turned-designer apparently wept tears of joy on discovering the gender of her fourth child with football star David.
The superstar couple have made no secret of wanting a daughter to add to their brood of boys.
Victoria, 36, told US interviewer Jay Leno in 2007: "I would love to have a little girl, I'm really girlie. I'd love to dress a little girl up".
The mother-of-three had a scan in January at the private Rivers Hospital, close to their family home in Sawbridgeworth, Hertfordshire, but it was too early to establish the gender of the child.
But then a scan at the 16-week mark revealed the news she was hoping for.
The source claimed that this will be Posh's last child and having a girl is "the icing on the cake for the Beckham clan".
They continued: "Victoria is 36 and David is thinking about life after retirement. For them, with a sister for their boys – who are growing up fast – this is the perfect family.
"All their dreams have come true. They couldn't have wished for more."
Victoria is said to be planning to deliver her baby in the US, at the Cedars-Sinai hospital, a favourite with the celebs.
It is expected she will have a Caesarean section, as she did with sons Brooklyn, Romeo and Cruz.
The couple are in Britain with their boys until the end of February as she prepares her latest collection for fashion week in London and New York.
Meanwhile, David is training with Tottenham Hotspur before returning to America for the start of the football season.
In other news, Victoria's former bandmate Emma Bunton has revealed she is expecting a baby boy.