Source: BridgeBum Spade Suit Heart Suit Diamond Suit club suit Baby Blackwood is a 3NT ace-asking bid after a major-suit fit is found. It is just like Blackwood, except that it’s one level lower. Example auctions:
Opener Responder
1Heart Suit 3Heart Suit
Opener Responder
1Spade Suit 2Heart Suit
3Heart Suit 3NT
In these auctions, it’s extremely unlikely that 3NT is needed as a natural bid. Hence the idea to save one level of bidding space. Some partnerships also play 1Heart Suit/Spade Suit : 3NT as Baby Blackwood, although I prefer to play 3NT as 13-15 HCP with 4333 shape. Rather than use Baby Blackwood in this situation, I find a Jacoby 2NT response more practical.

 Responses to 3NT

The responses to 3NT are the same as Blackwood.
4club suit Shows 0 or 4 aces
4Diamond Suit Shows 1 ace
4Heart Suit Shows 2 aces
4Spade Suit Shows 3 aces
Alternatively, Key Card responses can be used.

Rebids by the 3NT bidder

After hearing partner’s response, a 4NT rebid is a king-asking bid. The responses follow the same pattern as the 3NT responses where 5club suitshows 0 or 4 kings, etc.