A portrait of Ariantje Coeymans Verplanck standing in an interior is the first known full-length, life-sized portrait of a woman in American art history. 

"This large, imposing portrait is the earliest full-size portrait of a woman in colonial America. Ariaantje Coeymans was born in Albany in 1672. Her father was Barent Pieterse Coeymans, a wealthy merchant, landowner, and founder of Coeymans, New York. In 1723, at the age of 51, she married 28-year-old David Verplanck of Albany. This painting was probably commissioned as a wedding portrait. The necklace of corn kernel beads she wears may be a reference to the Native American corn ground in the Coeymans' gristmill". 

Source: http://mktgart.tripod.com/musingblog/index.blog/1837289/coeymans-history/

Per some sources, the 1718 date derived from notions about when her house was built but recent dendrochronology dates the house to 1700 not 1718, as previously thought.