narwhals are mammals so they are born alive and weigh 150 to 250 pounds.

SIZE
Narwhals can grow to be about 16 feet (4.9 m) long (not counting the tooth), and weigh about 1.8 tons (1.6 tonnes). Females are slightly smaller, averaging about 13 feet (4 m) long, and weighing 1 ton (0.9 tonnes). At birth, narwhals are about 5 feet (1.5 m) long and 175 pounds (80 kg).