CREATE TABLE table_29507 ("Period" TEXT, "Live births per year" TEXT, "Deaths per year" TEXT, "Natural change per year" TEXT, "CBR*" TEXT, "CDR*" TEXT, "NC*" TEXT, "TFR*" TEXT, "IMR*" FLOAT, "Life expectancy total" TEXT, "Life expectancy males" TEXT, "Life expectancy females" TEXT)

What period was there a 22.7 NC?
SELECT "Period" FROM table_29507 WHERE "NC*" = '22.7'
SELECT "Period" FROM "table_29507" WHERE "NC*" = '22.7'