The couple — who wed in Gloucestershire, England in June 2011 — are already parents to 13-month-old daughter Ethel Mary.
Her rep declined to confirm the pregnancy in July, but after stepping out several times with a burgeoning belly, no official confirmation was needed when 27-year-old Allen appeared radiant on the red carpet during the British Fashion Awards in November.
On New Year's Eve, Allen was anxious the tot would emerge soon, Tweeting she was "only planning on spending a few days of  pregnant. Unlike the last 3 years." And she said she had been sampling curries to try to force labor.