One says says that 1/3 of American women experience pregnancy before the age of 20 and doesn't refer to whether they are married or not.

The other says that 18.8% of women between the ages of 15 and 19 give birth in the US.

Neither of these support your claim that 1/3 of all American women/girls under the age of 18 are single mothers. Hope you will retract your claim.