Blue whales are pregnant for 10-12 months. The newborn calf is about 7.5 m long and weighs about 5.5 – 7.3 tonnes. A baby blue whale drinks about 225 litres (about enough to fill a bath) of its mother’s fat-laden milk (it is 40-50% fat) a day, gaining 3.7 kilograms an hour, until at age 8 months they are 15 m long and 22.5 tonnes! The mother and calf may stay together for a year or longer, until the calf is about 13 m long. Blue whales reach maturity at 10-15 years.