Du Plessis had returned home to be with his wife, Imari, putting a question mark against his participation in the first of a four-test series that starts on July 6.
But Thursday saw Du Plessis announce the birth of their child by tweeting: "Our biggest blessing yet came in a small package. So very grateful."
Opening batsman Dean Elgar led South Africa in their three-day warm-up match against the second-string England Lions in Worcester which started on Thursday.
If Du Plessis does return in time for the first test, opener Heino Kuhn, who replaced Stephen Cook in the squad, would like be South Africa's lone debutant at Lord's.
