Humpback whale breeding occurs mostly in the winter to early spring while near the surface and in warm, tropical waters. The gestation period is about 11-12 months and the calf is born tail first (this is normal for cetaceans) and near the surface in warm, shallow waters. The newborn instinctively swims to the surface within 10 seconds for its first breath; it is helped by its mother, using her flippers. Within 30 minutes of its birth the baby whale can swim. The newborn calf is about 14 feet long (4.3 m) and weighs about 2.5 tons (2.3 tonnes). Twins are extremely rare (about 1% of births); there is almost always one calf. The baby is nurtured with its mother's milk and is weaned in about 11 months. The mother and calf may stay together for a year or longer. Calves drink 100 pounds of milk each day. Humpback whales reach puberty at 4-7 years old, and maturity at 15 years. A calf is born to a female every 1-3 years.