Are mammals born alive or in a egg?

Alive

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Most mammals are born fully formed [infant version of the adults] and do not hatch from eggs.
Most mammals are born alive.

There are two egg-laying mammals. The platypus and the echidna are both egg-laying mammals, or monotremes.They are still classified as mammals because they feed their young on mothers' milk - a characteristic unique to mammals alone.

There are just three known species of egg-laying mammals, or monotremes. They are the platypus and short-beaked echidna (Tachyglossus aculeatus) of Australia, and the long-beaked echidna (Zaglosssus bruijni) of Papua New Guinea. The echidna is sometimes called the spiny anteater, and there are several sub-species of the long-beaked echidna: the Western long-beaked echidna, Sir David's long-beaked echidna and the Eastern long-beaked echidna.